Vic-20 memory configurations supported by emulator

The C128 team invented the GO64 command for compatibility.  Years ago I claimed to invent the GO128 command, and GO20 and some others.

My Commodore emulator (unified branch) that supports Windows and LCD targets including compact watch form factor, now supports an argument to the GO statement to specify the total memory to configure.

The Vic-20's memory map looks roughly like this.

//   (from emuvic20.cpp)
//   $0000-$03FF Low 1K RAM
//   $0400-$0FFF (3K RAM expansion)
//   $1000-$1DFF 3.5K RAM (for BASIC) [or alternate screen address]
//   $1E00-$1FFF 0.5K RAM (Screen characters)
//   $2000-$7FFF (24K RAM expansion)
//   $8000-$8FFF (Character ROM)
//   $9000-$9FFF (I/O)
//   $A000-$BFFF (8K Cartridge ROM, or RAM expansion)
//   $C000-$DFFF BASIC ROM
//   $E000-$FFFF KERNAL ROM

The Vic-20 was manufactured with 5K RAM, with only 3.5K available for BASIC, not counting the color RAM nybbles (half-bytes).

There is a missing hole of 3K RAM in lower memory, cartridge sold separately.   And you could also purchase 8K or 16K RAM cartridges.  And later on 24K and 32K solutions.   Up to a total of 35K RAM can be added into the memory map.

So specify the amount of RAM you want to add, and rounding down, the emulator will populate the appropriate memory.

But only 27.5K is available to BASIC programs.  1.5K is reserved for system use, the lower 3K RAM is only available to BASIC in some instances, and an upper 8K is reserved for cartridges and otherwise not contiguous, so also not available directly for BASIC use.

The VIC video chip (named VIC) can only address screen memory on the main board, so when both 3K and 8K memory expansions are populated, screen memory is directly in the middle, causing Commodore BASIC to ignore the lower 3K memory to prioritize for the larger contiguous memory configured after screen memory.  The 3K lower memory is only used directly when it is the only contiguous RAM expansion, such as with the separate 3K memory expansion cartridge and with the Super Expander that included 3K RAM expansion, but stole more memory for the programmable function keys, and for the graphics screen.

GO 20        (default, 3583 bytes free)
GO 20, 5    (minimum, 3583 bytes free)
GO 20, 8    (5K, +3K, 6655 bytes free)
GO 20, 13    (5K, +8K, 11775 bytes free)
GO 20, 16    (5K, +3K, +8K, 11775 bytes free)
GO 20, 21    (5K, +8K, +8K, 19967 bytes free)
GO 20, 24    (5K, +3K, +8K, +8K, 19967 bytes free)
GO 20, 29    (5K, +8K, +8K, +8K, 28159 bytes free)
GO 20, 32    (5K, +3K, +8K, +8K, +8K, 28159 bytes free)
GO 20, 37    (5K, +8K, +8K, +8K, +8K, 28159 bytes free)
GO 20, 40    (5K, +3K, +8K, +8K, +8K, +8K, 28159 bytes free)

Note that many of these show the same number of bytes free even though extra memory has been added. This again is because only contiguous RAM can be configured as available to BASIC.  The extra RAM is available for PEEKs, POKEs, and other machine language programs.

If a number out of range is entered, it is effectively rounded down to a valid configuration.  For example GO 20, 64 will configure with 40K RAM.   But 5 is the minimum, so GO 20,0 will configure 5K RAM.

Technically the Vic-20 tests for and can handle memory added in smaller configurations, but that's not commonly present in reality, and not how I implemented it.   Also I only implemented adding 8K memory forward in the map (with the exception of the 3K coming and going based on math), and not configuring each section separately.  Going with a total RAM value is a simpler concept to implement and understand.

Please enjoy the Vic-20 with extra RAM!