Editing IBM 1620 (section)

==Architecture==
===Memory===
The IBM 1620 Model I was a variable "[[word (data type)|word]]" length decimal ([[binary-coded decimal|BCD]]) computer using [[core memory]]. The Model I core could hold 20,000 decimal digits with each digit stored in six bits.<ref>"...  arranged as a 100x100 array of 12-bit locations, each holding 2 decimal digits. (Each decimal digit was encoded in binary using 6 bits.)"</ref><ref name=ChulaStat/> More memory could be added with the IBM 1623 Storage Unit, Model 1 which held 40,000 digits, or the 1623 Model 2 which held 60,000.<ref name=IBM.intro59/>

The Model II deployed the IBM 1625 core-storage memory unit,<ref>{{cite book
|chapter-url=https://link.springer.com/content/pdf/10.1007/978-3-642-03757-3_16.pdf |archive-url=https://web.archive.org/web/20171222052358/https://link.springer.com/content/pdf/10.1007/978-3-642-03757-3_16.pdf |archive-date=2017-12-22 |url-status=live   |author=Oddur Benediktsson  
|title=History of Nordic Computing 2  
|volume=303  
|pages=149–155  
|doi=10.1007/978-3-642-03757-3_16  
|series=IFIP Advances in Information and Communication Technology  
|year=2009  
|isbn=978-3-642-03756-6  
|chapter=FORTRAN II – the First Computer Language Used at the University of Iceland  
}}</ref><ref name=NordBook>{{cite book
|title=History of Nordic Computing 2: Second IFIP WG 9.7 Conference
|url=https://books.google.com/books?isbn=3642037577  |isbn=978-3642037573  |date=2009
|author1=John Impagliazzo   |author2=Timo Järvi  |author3=Petri Paju}}</ref> whose memory cycle time was halved by using faster cores, compared to the Model I's (internal or 1623 memory unit): to 10&nbsp;μs (i.e., the cycle speed was raised to 100&nbsp;kHz).

While the five-digit addresses of either model could have addressed 100,000 decimal digits, no machine larger than 60,000 decimal digits was ever marketed.<ref>Although there are descriptions of a single 100,000-digit machine, designed in the late 1960s, using heavily modified hardware.</ref>
<!-- see TALK page, "100,000 digit 1620" -->

===Memory access===
Memory was accessed two decimal digits at the same time (even-odd digit pair for numeric data or one ''[[alphanumeric|alphameric]]'' character for text data). Each decimal digit was six bits, composed of an odd parity '''C'''heck bit, a '''F'''lag bit, and four BCD bits for the value of the digit in the following format:<ref name=Physics>"The main memory was logically arranged as 20,000 6-bit words. Each word comprised four BCD data bits, a "flag" bit, and an odd parity check bit. Though this was its logical arrangement, physically memory was a 100x100 array of 12-bit ferrite core words, which causes a few quirks in the instruction set. All instructions occupied 12 consecutive digits of memory, and were required to start at an even address so that the two-digit opcode could be read in one 12-bit physical word." {{cite web |url=https://www.physics.smu.edu/fattarus/ibm1620.html |title=The IBM 1620 Model 1{{snd}} Physics @ SMU}}</ref>

   '''C F 8 4 2 1'''

The '''F'''lag bit had several uses:

* In the least significant digit it was set to indicate a negative number ([[sign-and-magnitude|signed magnitude]]).
* It was set to mark the most significant digit of a number (''[[word mark (computer hardware)|word mark]]'').
* In the least significant digit of five-digit addresses it was set for [[addressing mode#Memory indirect|indirect address]]ing (an option on the [[#Model I|Model I]], standard on the 1620 Model II). Multi-level indirection<ref name=IBM.intro59/> could be used (the machine could even be put into an infinite indirect addressing loop).
* In the middle three digits of five-digit addresses (on the [[IBM 1620 Model II|1620 II]]) they were set to select one of seven [[index register]]s.

In addition to the valid BCD digit values there were three ''special'' digit values (these could {{em|not}} be used in calculations):

   C F 8 4 2 1
       1 0 1 0 {{snd}} [[Recordmark character|Record Mark]] (right most end of record, prints as a [[double dagger]] symbol, ‡)
       1 1 0 0 {{snd}} Numeric Blank (blank for punched card output formatting)
       1 1 1 1 {{snd}} [[Groupmark character|Group Mark]] (right most end of a group of records for disk I/O)

[[Instruction set|Instructions]] were fixed length (12 decimal digits), consisting of a two-digit "[[opcode|op code]]", a five-digit "P Address" (usually the ''destination'' address), and a five-digit "Q Address" (usually the ''source'' address or the ''source'' immediate value). Some instructions, such as the B (branch) instruction, used only the P Address, and later smart assemblers included a "B7" instruction that generated a seven-digit branch instruction (op code, P address, and one extra digit because the next instruction had to start on an even-numbered digit).

[[Fixed-point arithmetic|Fixed-point]] data "words" could be any size from two decimal digits up to all of memory not used for other purposes.

[[Floating-point arithmetic|Floating-point]] data "words" (using the hardware [[floating-point arithmetic|floating-point]] option) could be any size from 4 decimal digits up to 102 decimal digits (2 to 100 digits for the [[Significand|mantissa]] and two digits for the [[exponent]]).

The Fortran II compiler offered limited access to this flexibility via a "Source Program Control Card" preceding the Fortran source in a fixed format:

 *ffkks

The * in column one, ''ff''  the number of digits for the mantissa of floating-point numbers (allowing 02 to 28), ''kk'' the number of digits for fixed-point numbers (allowing 04 to 10) and ''s'' is to specify the memory size of the computer to run the code if not the current computer: 2, 4, or 6 for memories of 20,000 or 40,000 or 60,000 digits.

The machine had no programmer-accessible registers: all operations were memory to memory (including the [[index register]]s of the [[IBM 1620 Model II|1620 II]]).

:''See [[#Architectural difficulties|Architectural difficulties]] section''

===Character and op codes===
{{Contains special characters
|special = uncommon [[Unicode]] characters
|fix     = Help:Multilingual support
|image   = Replacement character.svg
|link    = Specials (Unicode block)#Replacement character
|alt     = <?>
|compact = yes
}}
The table below lists Alphameric mode characters (and op codes).

{{hidden begin
|title = Table of character and op codes
|titlestyle = background:#ccccff;
}}

{{Clear}}
{| class="wikitable"
|-
! rowspan=2 | [[Binary-coded decimal#IBM and BCD|BCD]] character
! colspan=2 | Typewriter
! Printer
! colspan=2 | [[paper tape|Tape]]
! colspan=2 | &nbsp;Card&nbsp;
! colspan=2 | [[Magnetic-core memory|Core]]
! rowspan=2 | MNEMONIC and operation
! rowspan=2 | ''Definition'' and notes
|-
! In || Out || Out || In || Out || In || Out || Even || Odd
|-
| style="text-align:center;"| Blank
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| &nbsp;
| <code>C</code>
| <code>C</code>
| &nbsp;
| &nbsp;
| <code></code>
| <code>&nbsp;</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>1</code>
| FADD Floating Add
| Optional special feature.
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>2&nbsp;</code>
| FSUB Floating Subtract
| Optional special feature.
|-
| style="text-align:center;"| .
| style="text-align:center;"| .
| style="text-align:center;"| .
| style="text-align:center;"| .
| <code>&nbsp;X0&nbsp;8&nbsp;21</code>
| <code>&nbsp;X0&nbsp;8&nbsp;21</code>
| 12-3-8<br/>12-1-2-8
| 12-3-8
| <code></code>
| <code>21</code>
| FMUL Floating Multiply
| Optional special feature.
|-
| style="text-align:center;"| )
| style="text-align:center;"| )
| style="text-align:center;"| )
| style="text-align:center;"| )
| <code>&nbsp;X0C84</code>
| <code>&nbsp;X0C84</code>
| 12-4-8
| 12-4-8
| <code></code>
| <code>4</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>4&nbsp;1</code>
| FSL Floating Shift Left
| Optional special feature.
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>42&nbsp;</code>
| TFL Transmit Floating
| Optional special feature.
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>421</code>
| BTFL Branch and Transmit Floating
| Optional special feature.
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>&nbsp;8</code>
| FSR Floating Shift Right
| Optional special feature.
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code></code>
| <code>&nbsp;81</code>
| FDIV Floating Divide
| Optional special feature.
|-
| style="text-align:center;"| +
| style="text-align:center;"| +
| style="text-align:center;"| +
| style="text-align:center;"| +
| <code>&nbsp;X0C</code>
| <code>&nbsp;X0C</code>
| 12
| 12
| <code>1</code>
| <code>&nbsp;</code>
| BTAM Branch and Transmit Address Immediate
| (Model II)
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>1</code>
| AM Add Immediate
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>2&nbsp;</code>
| SM Subtract Immediate
| &nbsp;
|-
| style="text-align:center;"| $
| style="text-align:center;"| $
| style="text-align:center;"| $
| style="text-align:center;"| $
| <code>&nbsp;X&nbsp;C8&nbsp;21</code>
| <code>&nbsp;X&nbsp;C8&nbsp;21</code>
| 11-3-8<br/>11-1-2-8
| 11-3-8
| <code>1</code>
| <code>21</code>
| MM Multiply Immediate
| &nbsp;
|-
| style="text-align:center;"| *
| style="text-align:center;"| *
| style="text-align:center;"| *
| style="text-align:center;"| *
| <code>&nbsp;X84</code>
| <code>&nbsp;X84</code>
| 11-4-8
| 11-4-8
| <code>1</code>
| <code>4</code>
| CM Compare Immediate
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>4&nbsp;1</code>
| TDM Transmit Digit Immediate
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>42&nbsp;</code>
| TFM Transmit Field Immediate
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>421</code>
| BTM Branch and Transmit Immediate
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>&nbsp;8</code>
| LDM Load Dividend Immediate
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>1</code>
| <code>&nbsp;81</code>
| DM Divide Immediate
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| -
| style="text-align:center;"| -
| style="text-align:center;"| -
| style="text-align:center;"| -
| <code>&nbsp;X</code>
| <code>&nbsp;X</code>
| 11
| 11
| <code>2&nbsp;</code>
| <code>&nbsp;</code>
| BTA Branch and Transmit Address
| (Model II)
|-
| style="text-align:center;"| /
| style="text-align:center;"| /
| style="text-align:center;"| /
| style="text-align:center;"| /
| <code>0C1</code>
| <code>0C1</code>
| 0-1
| 0-1
| <code>2&nbsp;</code>
| <code>1</code>
| A Add
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>2&nbsp;</code>
| S Subtract
| &nbsp;
|-
| style="text-align:center;"|,
| style="text-align:center;"|,
| style="text-align:center;"|,
| style="text-align:center;"|,
| <code>0C8&nbsp;21</code>
| <code>0C8&nbsp;21</code>
| 0-3-8<br/>0-1-2-8
| 0-3-8
| <code>2&nbsp;</code>
| <code>21</code>
| M Multiply
| &nbsp;
|-
| style="text-align:center;"| (
| style="text-align:center;"| (
| style="text-align:center;"| (
| style="text-align:center;"| (
| <code>0&nbsp;84</code>
| <code>0&nbsp;84</code>
| 0-4-8
| 0-4-8
| <code>2&nbsp;</code>
| <code>4</code>
| C Compare
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>4&nbsp;1</code>
| TD Transmit Digit
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>42&nbsp;</code>
| TF Transmit Field
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>421</code>
| BT Branch and Transmit
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>&nbsp;8</code>
| LD Load Dividend
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>2&nbsp;</code>
| <code>&nbsp;81</code>
| D Divide
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>&nbsp;</code>
| TRNM Transmit Record No RM
| (Model II)
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>1</code>
| TR Transmit Record
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>2&nbsp;</code>
| SF Set Flag
| &nbsp;
|-
| style="text-align:center;"| =
| style="text-align:center;"| =
| style="text-align:center;"| =
| style="text-align:center;"| =
| <code>8&nbsp;21</code>
| <code>8&nbsp;21</code>
| 3-8<br/>1-2-8
| 3-8
| <code>21</code>
| <code>21</code>
| CF Clear Flag
| &nbsp;
|-
| style="text-align:center;"| @
| style="text-align:center;"| @
| style="text-align:center;"| @
| style="text-align:center;"| @
| <code>C84</code>
| <code>C84</code>
| 4-8
| 4-8
| <code>21</code>
| <code>4</code>
| K Control (I/O device)
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>4&nbsp;1</code>
| DN Dump Numeric
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>42&nbsp;</code>
| RN Read Numeric
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>421</code>
| RA Read Alphameric
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>&nbsp;8</code>
| WN Write Numeric
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>21</code>
| <code>&nbsp;81</code>
| WA Write Alphameric
| &nbsp;
|-
| style="text-align:center;"| A
| style="text-align:center;"| A
| style="text-align:center;"| A
| style="text-align:center;"| A
| <code>&nbsp;X01</code>
| <code>&nbsp;X01</code>
| 12-1
| 12-1
| <code>&nbsp;4</code>
| <code>1</code>
| NOP No Operation
| &nbsp;
|-
| style="text-align:center;"| B
| style="text-align:center;"| B
| style="text-align:center;"| B
| style="text-align:center;"| B
| <code>&nbsp;X02&nbsp;</code>
| <code>&nbsp;X02&nbsp;</code>
| 12-2
| 12-2
| <code>&nbsp;4</code>
| <code>2&nbsp;</code>
| BB Branch Back
| &nbsp;
|-
| style="text-align:center;"| C
| style="text-align:center;"| C
| style="text-align:center;"| C
| style="text-align:center;"| C
| <code>&nbsp;X0C21</code>
| <code>&nbsp;X0C21</code>
| 12-3<br/>12-1-2
| 12-3
| <code>&nbsp;4</code>
| <code>21</code>
| BD Branch On Digit
| &nbsp;
|-
| style="text-align:center;"| D
| style="text-align:center;"| D
| style="text-align:center;"| D
| style="text-align:center;"| D
| <code>&nbsp;X04</code>
| <code>&nbsp;X04</code>
| 12-4
| 12-4
| <code>&nbsp;4</code>
| <code>4</code>
| BNF Branch No Flag
| &nbsp;
|-
| style="text-align:center;"| E
| style="text-align:center;"| E
| style="text-align:center;"| E
| style="text-align:center;"| E
| <code>&nbsp;X0C&nbsp;4&nbsp;1</code>
| <code>&nbsp;X0C&nbsp;4&nbsp;1</code>
| 12-5<br/>12-1-4
| 12-5
| <code>&nbsp;4</code>
| <code>4&nbsp;1</code>
| BNR Branch No Record Mark
| &nbsp;
|-
| rowspan="2" style="text-align:center;"| F
| rowspan="2" style="text-align:center;"| F
| rowspan="2" style="text-align:center;"| F
| rowspan="2" style="text-align:center;"| F
| rowspan=2 | <code>&nbsp;X0C&nbsp;42&nbsp;</code>
| rowspan=2 | <code>&nbsp;X0C&nbsp;42&nbsp;</code>
| rowspan=2 | 12-6<br/>12-2-4
| rowspan=2 | 12-6
| rowspan=2 | <code>&nbsp;4</code>
| rowspan=2 | <code>42&nbsp;</code>
| BI Branch Indicator
| &nbsp;
|-
| UMK Unmask<br/>MK Mask
| 1710 interrupt feature. Modifiers in Q field.
|-
| rowspan="2" style="text-align:center;"| G
| rowspan="2" style="text-align:center;"| G
| rowspan="2" style="text-align:center;"| G
| rowspan="2" style="text-align:center;"| G
| rowspan=2 | <code>&nbsp;X0421</code>
| rowspan=2 | <code>&nbsp;X0421</code>
| rowspan=2 | 12-7<br/>12-1-2-4
| rowspan=2 | 12-7
| rowspan=2 | <code>&nbsp;4</code>
| rowspan=2 | <code>421</code>
| BNI Branch No Indicator
| &nbsp;
|-
| BO Branch Out<br/>BOLD Branch Out and Load
| 1710 interrupt feature. Modifiers in Q field.
|-
| style="text-align:center;"| H
| style="text-align:center;"| H
| style="text-align:center;"| H
| style="text-align:center;"| H
| <code>&nbsp;X0&nbsp;8</code>
| <code>&nbsp;X0&nbsp;8</code>
| 12-8
| 12-8
| <code>&nbsp;4</code>
| <code>&nbsp;8</code>
| H Halt
| &nbsp;
|-
| style="text-align:center;"| I
| style="text-align:center;"| I
| style="text-align:center;"| I
| style="text-align:center;"| I
| <code>&nbsp;X0C81</code>
| <code>&nbsp;X0C81</code>
| 12-9<br/>12-1-8
| 12-9
| <code>&nbsp;4</code>
| <code>&nbsp;81</code>
| B Branch
| &nbsp;
|-
| style="text-align:center;"| -0
| style="text-align:center;"| N/A
| style="text-align:center;"| -
| style="text-align:center;"| -
| style="text-align:center;"| N/A
| <code>&nbsp;X</code>
| 11-0
| 11-0
| <code>&nbsp;4&nbsp;1</code>
| <code>&nbsp;</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| J<br/>-1
| style="text-align:center;"| J
| style="text-align:center;"| J
| style="text-align:center;"| J
| <code>&nbsp;X&nbsp;C1</code>
| <code>&nbsp;X&nbsp;C1</code>
| 11-1
| 11-1
| <code>&nbsp;4&nbsp;1</code>
| <code>1</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| K<br/>-2
| style="text-align:center;"| K
| style="text-align:center;"| K
| style="text-align:center;"| K
| <code>&nbsp;X&nbsp;C2&nbsp;</code>
| <code>&nbsp;X&nbsp;C2&nbsp;</code>
| 11-2
| 11-2
| <code>&nbsp;4&nbsp;1</code>
| <code>2&nbsp;</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| L<br/>-3
| style="text-align:center;"| L
| style="text-align:center;"| L
| style="text-align:center;"| L
| <code>&nbsp;X21</code>
| <code>&nbsp;X21</code>
| 11-3<br/>11-1-2
| 11-3
| <code>&nbsp;4&nbsp;1</code>
| <code>21</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| M<br/>-4
| style="text-align:center;"| M
| style="text-align:center;"| M
| style="text-align:center;"| M
| <code>&nbsp;X&nbsp;C&nbsp;4</code>
| <code>&nbsp;X&nbsp;C&nbsp;4</code>
| 11-4
| 11-4
| <code>&nbsp;4&nbsp;1</code>
| <code>4</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| N<br/>-5
| style="text-align:center;"| N
| style="text-align:center;"| N
| style="text-align:center;"| N
| <code>&nbsp;X4&nbsp;1</code>
| <code>&nbsp;X4&nbsp;1</code>
| 11-5<br/>11-1-4
| 11-5
| <code>&nbsp;4&nbsp;1</code>
| <code>4&nbsp;1</code>
| BNG Branch No Group Mark
| Optional special feature.
|-
| style="text-align:center;"| O<br/>-6
| style="text-align:center;"| O
| style="text-align:center;"| O
| style="text-align:center;"| O
| <code>&nbsp;X42&nbsp;</code>
| <code>&nbsp;X42&nbsp;</code>
| 11-6<br/>11-2-4
| 11-6
| <code>&nbsp;4&nbsp;1</code>
| <code>42&nbsp;</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| P<br/>-7
| style="text-align:center;"| P
| style="text-align:center;"| P
| style="text-align:center;"| P
| <code>&nbsp;X&nbsp;C&nbsp;421</code>
| <code>&nbsp;X&nbsp;C&nbsp;421</code>
| 11-7<br/>11-1-2-4
| 11-7
| <code>&nbsp;4&nbsp;1</code>
| <code>421</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| Q<br/>-8
| style="text-align:center;"| Q
| style="text-align:center;"| Q
| style="text-align:center;"| Q
| <code>&nbsp;X&nbsp;C8</code>
| <code>&nbsp;X&nbsp;C8</code>
| 11-8
| 11-8
| <code>&nbsp;4&nbsp;1</code>
| <code>&nbsp;8</code>
|WAIT Wait for Interrupt 
|1710 interrupt feature.
|-
| style="text-align:center;"| R<br/>-9
| style="text-align:center;"| R
| style="text-align:center;"| R
| style="text-align:center;"| R
| <code>&nbsp;X81</code>
| <code>&nbsp;X81</code>
| 11-9<br/>11-1-8
| 11-9
| <code>&nbsp;4&nbsp;1</code>
| <code>&nbsp;81</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>&nbsp;42&nbsp;</code>
| <code>&nbsp;</code>
| BS Branch and Select
| (Model II)
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>&nbsp;42&nbsp;</code>
| <code>1</code>
| BX Branch and Modify Index Register
| Optional special feature (Model II).
|-
| style="text-align:center;"| S
| style="text-align:center;"| S
| style="text-align:center;"| S
| style="text-align:center;"| S
| <code>0C2&nbsp;</code>
| <code>0C2&nbsp;</code>
| 0-2
| 0-2
| <code>&nbsp;42&nbsp;</code>
| <code>2&nbsp;</code>
| BXM Branch <wbr/>and Modify Index Register Immediate
| Optional special feature (Model II).
|-
| style="text-align:center;"| T
| style="text-align:center;"| T
| style="text-align:center;"| T
| style="text-align:center;"| T
| <code>021</code>
| <code>021</code>
| 0-3<br/>0-1-2
| 0-3
| <code>&nbsp;42&nbsp;</code>
| <code>21</code>
| BCX Branch Conditionally<wbr/> and Modify Index Register
| Optional special feature (Model II).
|-
| style="text-align:center;"| U
| style="text-align:center;"| U
| style="text-align:center;"| U
| style="text-align:center;"| U
| <code>0C&nbsp;4</code>
| <code>0C&nbsp;4</code>
| 0-4
| 0-4
| <code>&nbsp;42&nbsp;</code>
| <code>4</code>
| BCXM Branch Conditionally <wbr/>and Modify Index Register Immediate
| Optional special feature (Model II).
|-
| style="text-align:center;"| V
| style="text-align:center;"| V
| style="text-align:center;"| V
| style="text-align:center;"| V
| <code>04&nbsp;1</code>
| <code>04&nbsp;1</code>
| 0-5<br/>0-1-4
| 0-5
| <code>&nbsp;42&nbsp;</code>
| <code>4&nbsp;1</code>
| BLX Branch and Load Index Register
| Optional special feature (Model II).
|-
| style="text-align:center;"| W
| style="text-align:center;"| W
| style="text-align:center;"| W
| style="text-align:center;"| W
| <code>042&nbsp;</code>
| <code>042&nbsp;</code>
| 0-6<br/>0-2-4
| 0-6
| <code>&nbsp;42&nbsp;</code>
| <code>42&nbsp;</code>
| BLXM Branch <wbr/>and Load Index Register Immediate
| Optional special feature (Model II).
|-
| style="text-align:center;"| X
| style="text-align:center;"| X
| style="text-align:center;"| X
| style="text-align:center;"| X
| <code>0C&nbsp;421</code>
| <code>0C&nbsp;421</code>
| 0-7<br/>0-1-2-4
| 0-7
| <code>&nbsp;42&nbsp;</code>
| <code>421</code>
| BSX Branch and Store Index Register
| Optional special feature (Model II).
|-
| style="text-align:center;"| Y
| style="text-align:center;"| Y
| style="text-align:center;"| Y
| style="text-align:center;"| Y
| <code>0C8</code>
| <code>0C8</code>
| 0-8
| 0-8
| <code>&nbsp;42&nbsp;</code>
| <code>&nbsp;8</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| Z
| style="text-align:center;"| Z
| style="text-align:center;"| Z
| style="text-align:center;"| Z
| <code>0&nbsp;81</code>
| <code>0&nbsp;81</code>
| 0-9<br/>0-1-8
| 0-9
| <code>&nbsp;42&nbsp;</code>
| <code>&nbsp;81</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| <code>0&nbsp;</code>
| <code>0&nbsp;</code>
| 0<br/>12-0
| 0
| <code>&nbsp;421</code>
| <code>&nbsp;</code>
| MA Move Address
| Optional special feature (Model II).
|-
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| <code>1</code>
| <code>1</code>
| 1
| 1
| <code>&nbsp;421</code>
| <code>1</code>
| MF Move Flag
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| <code>2&nbsp;</code>
| <code>2&nbsp;</code>
| 2
| 2
| <code>&nbsp;421</code>
| <code>2&nbsp;</code>
| TNS Transmit Numeric Strip
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| <code>C21</code>
| <code>C21</code>
| 3
| 3
| <code>&nbsp;421</code>
| <code>21</code>
| TNF Transmit Numeric Fill
| Optional special feature (Model I).<br/>Standard (Model II).
|-
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| <code>&nbsp;4</code>
| <code>&nbsp;4</code>
| 4
| 4
| <code>&nbsp;421</code>
| <code>4</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| <code>C&nbsp;4&nbsp;1</code>
| <code>C&nbsp;4&nbsp;1</code>
| 5
| 5
| <code>&nbsp;421</code>
| <code>4&nbsp;1</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| <code>C&nbsp;42&nbsp;</code>
| <code>C&nbsp;42&nbsp;</code>
| 6
| 6
| <code>&nbsp;421</code>
| <code>42&nbsp;</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| <code>&nbsp;421</code>
| <code>&nbsp;421</code>
| 7
| 7
| <code>&nbsp;421</code>
| <code>421</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| <code>8</code>
| <code>8</code>
| 8
| 8
| <code>&nbsp;421</code>
| <code>&nbsp;8</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| <code>C81</code>
| <code>C81</code>
| 9
| 9
| <code>&nbsp;421</code>
| <code>&nbsp;81</code>
| &nbsp;
| &nbsp;
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>4</code>
| SA Select Address<br/>SACO Select Address, Contact Operate<br/>SAOS Select Analog Output Signal
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>42&nbsp;</code>
| SLTA Select TAS<br/>SLAR Select ADC Register<br/>SLTC Select Real-Time Clock<br/>SLIC Select Input Channel<br/>SLCB Select Contact Block<br/>SLME Select Manual Entry
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>F&nbsp;42&nbsp;</code>
| RNIC Read Numeric Input Channel
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>F&nbsp;421</code>
| RAIC Read Alphameric Input Channel
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>&nbsp;8</code>
| WNOC Write Numeric Output Channel
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>8</code>
| <code>&nbsp;81</code>
| WAOC Write Alphameric Output Channel
| 1710 feature. Modifiers in Q field
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>&nbsp;</code>
| BBT Branch on Bit
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>1</code>
| BMK Branch on Mask
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>2&nbsp;</code>
| ORF OR to Field
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>21</code>
| ANDF AND to Field
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>4</code>
| CPLF Complement Octal Field
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>4&nbsp;1</code>
| EORF Exclusive OR to Field
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>42&nbsp;</code>
| OTD Octal to Decimal Conversion
| Optional special feature (Model II).
|-
| style="text-align:center;"| invalid
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| Ж ❚
| colspan=5 | &nbsp;
| <code>81</code>
| <code>421</code>
| DTO Decimal to Octal Conversion
| Optional special feature (Model II).
|-
| style="text-align:center;"| RM
| style="text-align:center;"| ‡
| style="text-align:center;"| (Stop)
| style="text-align:center;"| (Stop)
| <code>0&nbsp;8&nbsp;2&nbsp;</code>
| <code>E</code><br/>(Stop)
| 0-2-8
| 0-2-8
| <code></code>
| <code>&nbsp;8&nbsp;2&nbsp;</code>
| &nbsp;
| ''Record Mark''
|-
| style="text-align:center;"| GM
| style="text-align:center;"| [[File:IBM 1401 Group Mark.GIF|12px]]
| style="text-align:center;"| (Stop)
| style="text-align:center;"| (Stop)
| <code>0&nbsp;8421</code>
| <code>E</code><br/>(Stop)
| 0-7-8
| 1-2-4-8
| <code></code>
| <code>&nbsp;8421</code>
| &nbsp;
| ''Group Mark''
|}
{{Clear left}}

The table below lists numeric mode characters.
{| class="wikitable"
|-
! rowspan=2 | Character
! colspan=2 | Typewriter
! colspan=2 | Printer
! colspan=2 | [[paper tape|Tape]]
! colspan=2 | &nbsp;Card&nbsp;
! rowspan=2 | [[Magnetic-core memory|Core]]
! rowspan=2 | ''Definition'' and notes
|-
! In || Out || Out || Dump || In || Out || In || Out
|-
| style="text-align:center;"| Blank
| style="text-align:center;"| &nbsp;
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| <code>C</code>
| <code>0&nbsp;</code>
| &nbsp;
| 0
| <code>C&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| style="text-align:center;"| 0
| <code>0&nbsp;</code>
| <code>0&nbsp;</code>
| 0<br/>12-0<br/>12
| 0
| <code>C&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| style="text-align:center;"| 1
| <code>1</code>
| <code>1</code>
| 1<br/>12-1
| 1
| <code>&nbsp;1</code>
| &nbsp;
|-
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| style="text-align:center;"| 2
| <code>2&nbsp;</code>
| <code>2&nbsp;</code>
| 2<br/>12-1
| 2
| <code>2&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| style="text-align:center;"| 3
| <code>C21</code>
| <code>C21</code>
| 3<br/>12-3<br/>1-2<br/>12-1-2
| 3
| <code>C21</code>
| &nbsp;
|-
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| style="text-align:center;"| 4
| <code>&nbsp;4</code>
| <code>&nbsp;4</code>
| 4<br/>12-4
| 4
| <code>4</code>
| &nbsp;
|-
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| style="text-align:center;"| 5
| <code>C&nbsp;4&nbsp;1</code>
| <code>C&nbsp;4&nbsp;1</code>
| 5<br/>12-5<br/>1-4<br/>12-1-4
| 5
| <code>C4&nbsp;1</code>
| &nbsp;
|-
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| style="text-align:center;"| 6
| <code>C&nbsp;42&nbsp;</code>
| <code>C&nbsp;42&nbsp;</code>
| 6<br/>12-6<br/>2-4<br/>12-2-4
| 6
| <code>C42&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| style="text-align:center;"| 7
| <code>&nbsp;421</code>
| <code>&nbsp;421</code>
| 7<br/>12-7<br/>1-2-4<br/>12-1-2-4
| 7
| <code>421</code>
| &nbsp;
|-
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| style="text-align:center;"| 8
| <code>8</code>
| <code>8</code>
| 8<br/>12-8
| 8
| <code>8</code>
| &nbsp;
|-
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| style="text-align:center;"| 9
| <code>C81</code>
| <code>C81</code>
| 9<br/>12-9<br/>1-8<br/>12-1-8
| 9
| <code>C&nbsp;81</code>
| &nbsp;
|-
| style="text-align:center;"| -0
| style="text-align:center;"| <span style="text-decoration: overline">0</span>
| style="text-align:center;"| <span style="text-decoration: overline">0</span>
| style="text-align:center;"| -
| style="text-align:center;"| -
| <code>&nbsp;X</code><br/><code>&nbsp;X0C</code>
| <code>&nbsp;X</code>
| 11-0
| 11-0
| <code>&nbsp;F</code>
| &nbsp;
|-
| style="text-align:center;"| -1
| style="text-align:center;"| <span style="text-decoration: overline">1</span>
| style="text-align:center;"| <span style="text-decoration: overline">1</span>
| style="text-align:center;"| J
| style="text-align:center;"| J
| <code>&nbsp;X&nbsp;C1</code>
| <code>&nbsp;X&nbsp;C1</code>
| 11-1
| 11-1
| <code>CF1</code>
| &nbsp;
|-
| style="text-align:center;"| -2
| style="text-align:center;"| <span style="text-decoration: overline">2</span>
| style="text-align:center;"| <span style="text-decoration: overline">2</span>
| style="text-align:center;"| K
| style="text-align:center;"| K
| <code>&nbsp;X&nbsp;C2&nbsp;</code>
| <code>&nbsp;X&nbsp;C2&nbsp;</code>
| 11-2
| 11-2
| <code>CF2&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| -3
| style="text-align:center;"| <span style="text-decoration: overline">3</span>
| style="text-align:center;"| <span style="text-decoration: overline">3</span>
| style="text-align:center;"| L
| style="text-align:center;"| L
| <code>&nbsp;X21</code>
| <code>&nbsp;X21</code>
| 11-3<br/>11-1-2
| 11-3
| <code>&nbsp;F21</code>
| &nbsp;
|-
| style="text-align:center;"| -4
| style="text-align:center;"| <span style="text-decoration: overline">4</span>
| style="text-align:center;"| <span style="text-decoration: overline">4</span>
| style="text-align:center;"| M
| style="text-align:center;"| M
| <code>&nbsp;X&nbsp;C&nbsp;4</code>
| <code>&nbsp;X&nbsp;C&nbsp;4</code>
| 11-4
| 11-4
| <code>CF&nbsp;4</code>
| &nbsp;
|-
| style="text-align:center;"| -5
| style="text-align:center;"| <span style="text-decoration: overline">5</span>
| style="text-align:center;"| <span style="text-decoration: overline">5</span>
| style="text-align:center;"| N
| style="text-align:center;"| N
| <code>&nbsp;X4&nbsp;1</code>
| <code>&nbsp;X4&nbsp;1</code>
| 11-5<br/>11-1-4
| 11-5
| <code>&nbsp;F&nbsp;4&nbsp;1</code>
| &nbsp;
|-
| style="text-align:center;"| -6
| style="text-align:center;"| <span style="text-decoration: overline">6</span>
| style="text-align:center;"| <span style="text-decoration: overline">6</span>
| style="text-align:center;"| O
| style="text-align:center;"| O
| <code>&nbsp;X42&nbsp;</code>
| <code>&nbsp;X42&nbsp;</code>
| 11-6<br/>11-2-4
| 11-6
| <code>&nbsp;F&nbsp;42&nbsp;</code>
| &nbsp;
|-
| style="text-align:center;"| -7
| style="text-align:center;"| <span style="text-decoration: overline">7</span>
| style="text-align:center;"| <span style="text-decoration: overline">7</span>
| style="text-align:center;"| P
| style="text-align:center;"| P
| <code>&nbsp;X&nbsp;C&nbsp;421</code>
| <code>&nbsp;X&nbsp;C&nbsp;421</code>
| 11-7<br/>11-1-2-4
| 11-7
| <code>CF&nbsp;421</code>
| &nbsp;
|-
| style="text-align:center;"| -8
| style="text-align:center;"| <span style="text-decoration: overline">8</span>
| style="text-align:center;"| <span style="text-decoration: overline">8</span>
| style="text-align:center;"| Q
| style="text-align:center;"| Q
| <code>&nbsp;X&nbsp;C8</code>
| <code>&nbsp;X&nbsp;C8</code>
| 11-8
| 11-8
| <code>CF8</code>
| &nbsp;
|-
| style="text-align:center;"| -9
| style="text-align:center;"| <span style="text-decoration: overline">9</span>
| style="text-align:center;"| <span style="text-decoration: overline">9</span>
| style="text-align:center;"| R
| style="text-align:center;"| R
| <code>&nbsp;X81</code>
| <code>&nbsp;X81</code>
| 11-9<br/>11-1-8
| 11-9
| <code>&nbsp;F81</code>
| &nbsp;
|-
| style="text-align:center;"| RM
| style="text-align:center;"| ‡
| style="text-align:center;"| (Stop, WN)<br/>‡<br/>(DN)
| style="text-align:center;"| (Stop)
| style="text-align:center;"| ‡
| <code>0&nbsp;8&nbsp;2&nbsp;</code>
| <code>E</code><br/>(Stop, WN)<br/><code>0&nbsp;8&nbsp;2&nbsp;</code><br/>(DN)
| 0-2-8
| 0-2-8
| <code>C&nbsp;8&nbsp;2&nbsp;</code>
| ''Record Mark''<br/>On tape a WN punches EOL instead!
|-
| style="text-align:center;"| flag RM
| style="text-align:center;"| <span style="text-decoration: overline">‡</span>
| style="text-align:center;"| (Stop, WN)<br/><span style="text-decoration: overline">‡</span><br/>(DN)
| style="text-align:center;"| (Stop)
| style="text-align:center;"| W
| <code>&nbsp;X8&nbsp;2&nbsp;</code>
| <code>E</code><br/>(Stop, WN)<br/><code>&nbsp;X8&nbsp;2&nbsp;</code><br/>(DN)
| 11-2-8<br/>12-2-8
| 11-2-8
| <code>&nbsp;F8&nbsp;2&nbsp;</code>
| ''Flagged Record Mark''<br/>On tape a WN punches EOL instead!
|-
| style="text-align:center;"| EOL
| style="text-align:center;"| ‡
| style="text-align:center;"| (Stop, WN)<br/>‡<br/>(DN)
| style="text-align:center;"| (Stop)
| style="text-align:center;"| ‡
| <code>E</code>
| <code>E</code><br/>(WN)<br/><code>0&nbsp;8&nbsp;2&nbsp;</code><br/>(DN)
| 0-2-8
| 0-2-8
| <code>C&nbsp;8&nbsp;2&nbsp;</code>
| ''End of line'' Tape only.<br/>Note: In memory is a Record Mark!
|-
| style="text-align:center;"| GM
| style="text-align:center;"| [[File:IBM 1401 Group Mark.GIF|12px]]
| style="text-align:center;"| (Stop, WN)<br/>[[File:IBM 1401 Group Mark.GIF|12px]]<br/>(DN)
| style="text-align:center;"| (Stop)
| style="text-align:center;"| G
| <code>0&nbsp;8421</code>
| <code>0&nbsp;8421</code>
| 0-7-8
| 0-7-8
| <code>C&nbsp;8421</code>
| ''Group Mark''
|-
| style="text-align:center;"| flag GM
| style="text-align:center;"| <span style="text-decoration: overline">[[File:IBM 1401 Group Mark.GIF|12px]]</span>
| style="text-align:center;"| (Stop, WN)<br/><span style="text-decoration: overline">[[File:IBM 1401 Group Mark.GIF|12px]]</span><br/>(DN)
| style="text-align:center;"| (Stop)
| style="text-align:center;"| X
| <code>&nbsp;X8421</code>
| <code>&nbsp;X8421</code>
| 12-7-8
| 12-7-8
| <code>&nbsp;F8421</code>
| ''Flagged Group Mark''
|-
| style="text-align:center;"| NB
| style="text-align:center;"| @
| style="text-align:center;"| @
| style="text-align:center;"|
| style="text-align:center;"| @
| <code>C84</code>
| <code>C84</code>
| 4-8
|
| <code>C&nbsp;84</code>
| ''Numeric Blank''
|-
| style="text-align:center;"| flag NB
| style="text-align:center;"| <span style="text-decoration: overline">@</span>
| style="text-align:center;"| <span style="text-decoration: overline">@</span>
| style="text-align:center;"|
| style="text-align:center;"| *
| <code>&nbsp;X84</code>
| <code>&nbsp;X84</code>
| 11-4-8
|
| <code>&nbsp;F84</code>
| ''Flagged Numeric Blank''
|}
{{Clear left}}

{{hidden end}}

===Invalid character===
The [[IBM 1620 Model I|Model I]] used the [[Cyrillic script|Cyrillic character]] [[Zhe (Cyrillic)|Ж]] (pronounced zh) on the typewriter as a general purpose invalid character with correct parity (invalid parity being indicated with an overstrike "&ndash;"). In some 1620 installations it was called a SMERSH, as used in the [[James Bond]] novels that had become popular in the late 1960s. The [[IBM 1620 Model II|Model II]] used a new character ❚ (called "pillow") as a general purpose invalid character with correct parity.

===Architectural difficulties===
Although the IBM 1620's architecture was very popular in the scientific and engineering community, computer scientist [[Edsger Dijkstra]] pointed out several flaws in its design in EWD37, "A review of the IBM 1620 data processing system".<ref>[https://www.cs.utexas.edu/users/EWD/transcriptions/EWD00xx/EWD37.html E.W. Dijkstra Archive: A review of the IBM 1620 Data Processing System (EWD 37)]</ref> Among these are that the machine's Branch and Transmit instruction together with Branch Back allow only one level of nested subroutine call, forcing the programmer of any code with more than one level to decide where the use of this "feature" would be most effective. <!-- The branch-back address could NOT be saved or restored as there was no instruction to do that. --> He also showed how the machine's paper tape reading support could not properly read tapes containing record marks, since record marks are used to terminate the characters read in storage. One effect of this is that the 1620 cannot duplicate a tape with record marks in a straightforward way: when the record mark is encountered, the punch instruction punches an EOL character instead and terminates. However this was not a crippling problem:

* the data can be copied to the end of memory and punched verbatim with a DN instruction instead of WN
* tapes were usually duplicated [[offline]].

Most 1620 installations used the more convenient punched card input/output,<ref>{{cite web|url=http://www.textfiles.com/bitsavers/pdf/ibm/1620/Basic_Programming_Concepts_and_the_IBM_1620_Computer_1962.pdf |archive-url=https://web.archive.org/web/20150720184343/http://www.textfiles.com/bitsavers/pdf/ibm/1620/Basic_Programming_Concepts_and_the_IBM_1620_Computer_1962.pdf |title=Basic Programming Concepts and The IBM 1620 Computer |archive-date=2015-07-20 |quote=The punched card is the most widely used media for communication with machines}}</ref> rather than paper tape.

The successor to the 1620, the [[IBM 1130]],<ref>"Similar demands for small to medium scientific computers resulted in the IBM 1620
and its successor the IBM 1130." {{cite book
|url=https://books.google.com/books?isbn=1483268594   |isbn=978-1483268590 
|title=Computer Organization and Assembly Language Programming
|author1=James L. Peterson  |author2=Werner Rheinboldt   |date=2014}}</ref> was based on a totally different, 16-bit binary architecture. (The 1130 line retained one 1620 peripheral, the [[IBM 1627]] drum plotter.)