Editing IBM 1620 (section)

===Console===
[[File:IBM 1620 Memory address register display selector.jpg|thumb|IBM 1620 Memory address register display selector switch]]
The most important items on the 1620's console were a pair of buttons labeled ''Insert'' & ''Release'', and the console typewriter.
* Insert – Pressing this key with the computer in ''Manual'' mode reset the program counter (in the MARS core memory) to zero, switched the computer into ''Automatic'' and ''Insert'' modes, and simulated the execution of a Read Numeric from Typewriter to address zero (unlocked the typewriter keyboard, shifted the typewriter into numeric mode). Note: unlike a real Read Numeric from Typewriter, ''Insert'' mode would force a ''Release'' after 100 digits had been typed to prevent overwriting the arithmetic tables.
* Release – Pressing this key while doing a Read from the Typewriter terminated the Read, switched the computer into ''Manual'' mode, and locked the typewriter keyboard.
The typewriter is used for operator input/output, both as the main console control of the computer and for program controlled input/output. Later models of the typewriter had a special key marked ''R-S'' that combined the functions of the console ''Release'' & ''Start'' buttons (this would be considered equivalent to an ''Enter'' key on a modern keyboard). Note: several keys on the typewriter did not generate input characters, these included ''Tab'' and ''Return'' (the 1620s alphameric and numeric BCD character sets lacked character codes for these keys).

The next most important items on the console were the buttons labeled ''Start'', ''Stop-SIE'', and ''Instant Stop-SCE''.

* Start – Pressing this key with the computer in ''Manual'' mode switched the computer to ''Automatic'' mode (causing the computer to begin executing at the address in the program counter).
* Stop-SIE – Pressing this key with the computer in ''Automatic'' mode switched the computer to ''Manual'' mode when the currently executing instruction completes. Pressing this key with the computer in ''Manual'' mode switched the computer into ''Automatic'' mode for one instruction.
* Instant Stop-SCE – Pressing this key with the computer in ''Automatic'' mode switched the computer into ''Automatic/Manual'' mode at the end of the current memory cycle. Pressing this key with the computer in ''Manual'' or ''Automatic/Manual'' mode switched the computer into ''Automatic/Manual'' mode and executed one memory cycle.
For program debugging there were the buttons labeled ''Save'' & ''Display MAR''.
* Save – Pressing this key with the computer in ''Manual'' mode saved the program counter into another register in the MARS core memory and activated ''Save'' mode.
When a Branch Back instruction was executed in ''Save'' mode, it copied the saved value back to the program counter (instead of copying the return address register as it normally did) and deactivated ''Save'' mode.

This was used during debugging to remember where the program had been stopped to allow it to be resumed after the debugging instructions that the operator had typed on the typewriter had finished. Note: the MARS register used to save the program counter in was also used by the ''Multiply'' instruction, so this instruction and the ''Save'' mode were incompatible! However, there was no need to use multiply in debugging code, so this was not considered to be a problem.

* Display MAR – Pressing this key with the computer in ''Manual'' mode displayed the selected MARS register and the contents of the memory at that address on the console lamps.

{| class="wikitable"
|-
!"Breakpoint" procedure
!Notes
|-
|Press ''Stop-SIE''
|Stop the computer at the end of the current instruction.
|-
|Press ''Save''
|Save the address to resume execution at.
|-
|Press ''Insert''
|Unlocks typewriter keyboard and shifts into numeric mode.
|-
|Type 35xxxxx0010036xxxxx0010042<br/>&nbsp;
|xxxxx is the address that you plan to set the breakpoint at.
|-
|Press ''Release''
|Locks typewriter keyboard.
|-
|Press ''Start''
|Begin execution. Allow the 12 digit instruction to print out.
|-
|Press ''Release''
|Stops the Dump Numeric.
|-
|Press ''Start''
|Begin execution.
|-
|Type 48
|Replace the opcode of the instruction to "break" at with a Halt opcode.
|-
|Press ''Release''
|Locks typewriter keyboard.
|-
|Press ''Start''
|Resume execution. Wait until the computer halts at the "breakpoint".
|-
|Press ''Insert''
|Unlocks typewriter keyboard and shifts into numeric mode.
|-
|Type 36xxxxx0010049xxxxx<br/>&nbsp;
|xxxxx is the address that you previously set the breakpoint at, you are now going to clear it.
|-
|Press ''Release''
|Locks typewriter keyboard.
|-
|Press ''Start''
|Begin execution.
|-
|Type oo
|oo is the 2 digit opcode the original 12 digit instruction previously printed out.
|-
|Press ''Release''
|Locks typewriter keyboard.
|-
|Press ''Stop-SIE''
|The machine is now ready to resume execution from the location of the (now cleared) "breakpoint". You can perform any required debugging actions now, before continuing.
|}

All of main memory could be cleared from the console by entering and executing a transfer instruction from address to address +1, this would overwrite any word mark, that would normally stop a transfer instruction, and wrap around at the end of memory. After a moment, pressing Stop would stop the transfer instruction and memory would be cleared.