v3.1 VIC-20 C64 C16 Plus/4 C128 X16

The CHARSET command tells the display chip where to look for the character set.

CHARSET [RAM|ROM] <value>

Changing the character set is slightly different on each platform.

The command tells the VIC-II chip to look for the character set at <value> * $800, relative to selected the VIC bank (that is, 0 by default). The RAM and ROM setting has no effect. The VIC-II chip will always see the ROM when <value> is set to 2 or 3. The following example list all possible setups in Bank 0.

CHARSET 0 ' chars at $0000
CHARSET 1 ' chars at $0800
CHARSET 2 ' uppercase/graphic chars in ROM
CHARSET 3 ' lowercase/uppercase chars in ROM
CHARSET 4 ' chars at $2000
CHARSET 5 ' chars at $2800
CHARSET 6 ' chars at $3000
CHARSET 7 ' chars at $3800

The address is calculated as <value> * $0400. The VIC chip can either see ROM or RAM. It can't see the expansion RAM, therefore some settings make no sense. You basically have the following (more or less useful) options:

CHARSET ROM 0 ' uppercase with full graphic chars
CHARSET ROM 1 ' reversed uppercase and graphic chars
CHARSET ROM 2 ' lowercase/uppercase chars with some graphics
CHARSET ROM 3 ' reversed lowercase/uppercase chars with some graphics
CHARSET RAM 0 ' chars in RAM at $0000
CHARSET RAM 4 ' chars in RAM at $1000
CHARSET RAM 5 ' chars in RAM at $1400
CHARSET RAM 6 ' chars in RAM at $1800
CHARSET RAM 7 ' chars in RAM at $1C00

The address is calculated as <value> * $0400. The TED chip can basically see all RAM and ROM, so you have 128 possible settings here as <value> can range from 0 to 63 and both RAM and ROM settings work.

The X16 supports the following modes:

' Uploads the character set found 
' at $2000 from RAM to VRAM and uses it
CHARSET RAM $2000
' Sets ISO charset
CHARSET ROM 1
' Sets PET upper/graph charset
CHARSET ROM 2
' Sets PET lower/upper charset
CHARSET ROM 3