Editing VIC-20 (section)

=== Memory ===
[[File:4849 - VIC-1211A Super Expander w 3k RAM.JPG|thumb|A {{val|3|u=KB}} RAM expansion cartridge with BASIC extension ROM]]
[[File:4853 - VIC-1111 16K RAM For VIC-20 open.JPG|thumb|A {{val|16|u=KB}} RAM expansion cartridge with PCB view]]

The VIC-20 shipped with {{nowrap|5 KB}} [[random-access memory|RAM]], but {{nowrap|1.5 KB}} of this is used for the video display and dynamic aspects of the [[read-only memory|ROM]]-resident [[Commodore BASIC]] and [[KERNAL]] (a low-level operating system). Only 3,583 [[byte]]s of BASIC program memory for code and [[Variable (computer science)|variables]] are actually available on an unexpanded machine.

Unlike the PET, the VIC-20 does not include a [[Machine code monitor|machine language monitor]], but Commodore offered them on disk, tape, or cartridge, with several different [[executable]]s to load into various memory locations. The monitor programs were the same as the PET monitor but added a mini-[[Assembly language#Assembler|assembler]] instead of requiring the user to enter [[hexadecimal]] [[opcode]]s.

The VIC-20's RAM is expandable through the cartridge port via a RAM cartridge. RAM cartridges were available from Commodore in several sizes: {{val|3|u=KB}} (with or without an included "Super Expander" BASIC extension ROM), {{val|8|u=KB}}, and {{val|16|u=KB}}. The internal [[memory map]] is reorganized if you plug in {{val|8|u=KB}} and {{val|16|u=KB}} cartridges, leading to a situation where some programs only work if the right amount of memory is present (the most significant divide being between a machine with no or {{val|3|u=KB}} extra memory on one hand, and a machine with more extra memory on the other).<ref>The main issue was that BASIC could only see one contiguous chunk of memory. That is, in order for your expansion memory to be visible and useful to BASIC, it needs to be added next to the existing BASIC program area. However, the video buffer memory was hardwired to a position rather close to the default location of the BASIC program area, so expansion cartridges offering more than {{val|3|u=KB}} had to move the location where BASIC looked for its program.</ref>

Most expansion cartridges featured hardware [[DIP switch]]es, allowing the RAM to be enabled in user-selectable memory blocks. Since the VIC-20 was designed to use SRAM rather than [[Dynamic random-access memory|DRAM]], the system board has no provisions for DRAM refresh.  RAM expansion cartridges ultimately allowed adding up to {{val|24|u=KB}} to the BASIC user memory; together with the {{val|3.5|u=KB}} built-in user memory, this gave a maximum of {{val|27.5|u=KB}} for BASIC programs and variables. Memory not visible to BASIC could still be used by machine code programs.{{citation needed|date=March 2023}}

{| class="wikitable"
|+ Memory map<ref name="zi_mm">{{cite web|title=VIC-20 memory map (long)|url=http://www.zimmers.net/cbmpics/cbm/vic/memorymap.txt|date=19 September 2005|access-date=2013-05-20|publisher=zimmers.net}}</ref>
! Address<br/>(hexadecimal)    || Size<br/>&#91;in KB&#93; || Description || Cartridge<br>decoded<ref name="ca_pin">{{cite web|title=VIC 20 / Commodore 64 RS 232 |url=http://www.commodore.ca/manuals/pdfs/commodore_pet_vic-20_c64_port_pinouts.pdf|date=30 March 2011|access-date=2013-05-21|publisher=commodore.ca}}</ref>
|-
|align="right"| {{mono|0000}} <!--{{mono|03FF}}-->
|align="right"| 1.0 || RAM with jump vectors etc.      || 
|-
|align="right"| {{mono|0400}} <!--{{mono|1000}}-->
|align="right"| 3.0 || Expansion                       || *
|-
|align="right"| {{mono|1000}}<!--{{mono|1FFF}}-->
|align="right"| 4.0 || RAM for BASIC and screen        || 
|-
|align="right"| {{mono|2000}}<!--{{mono|3FFF}}-->
|align="right"| 8.0 || Expansion block 1               || *
|-
|align="right"| {{mono|4000}}<!--{{mono|5FFF}}-->
|align="right"| 8.0 || Expansion block 2               || *
|-
|align="right"| {{mono|6000}}<!--{{mono|7FFF}}-->
|align="right"| 8.0 || Expansion block 3               || *
|-
|align="right"| {{mono|8000}}<!--{{mono|8FFF}}-->
|align="right"| 4.0 || ROM character bitmap            || 
|-
|align="right"| {{mono|9000}}<!--{{mono|93FF}}-->
|align="right"| 1.0 || I/O for [[MOS Technology VIC|VIC]], [[MOS Technology 6522|6522]] VIA#1, 6522 VIA#2, block 0 || 
|-
|align="right"| {{mono|9400}}<!--{{mono|95FF}}-->
|align="right"| 0.5 || Used for color RAM when expansion RAM at block 1 || 
|-
|align="right"| {{mono|9600}}<!--{{mono|97FF}}-->
|align="right"| 0.5 || Color RAM (normally)            || 
|-
|align="right"| {{mono|9800}}<!--{{mono|9BFF}}-->
|align="right"| 1.0 || I/O block 2                     || *
|-
|align="right"| {{mono|9C00}}<!--{{mono|9FFF}}-->
|align="right"| 1.0 || I/O block 3                     || *
|-
|align="right"| {{mono|A000}}<!--{{mono|BFFF}}-->
|align="right"| 8.0 || Decoded for expansion ROM       || *
|-
|align="right"| {{mono|C000}}<!--{{mono|DFFF}}-->
|align="right"| 8.0 || ROM [[Commodore BASIC|BASIC]]   || 
|-
|align="right"| {{mono|E000}}<!--{{mono|FFFF}}-->
|align="right"| 8.0 || ROM [[KERNAL]]                  || 
|}