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360 AMD Geode™ LX Processors Data Book
Display Controller Register Descriptions
33234H
6.6.18.6 VGA Character Map Select
Character Map A (bits [5,3:2]) and Character Map B (bits [4,1:0]) determine which font tables are used when displaying a
character in text mode. When bit 3 of the character's attribute = 1, Character Map A is used; when bit 3 of the character's
attribute = 0, Character Map B is used. The font tables are stored in the 64 KB in map 2. There are eight font tables. The
character map codes select the font tables as shown in Table 6-52.
6.6.18.7 VGA Memory Mode
Index 03h
Typ e R /W
Reset Value xxh
VGA Character Map Select Register Bit Descriptions
Bit Name Description
7:6 RSVD Reserved. Write as read.
5 CHAR_AZ Character Map A bit 2.
4 CHAR_BZ Character Map B bit 2.
3:2 CHAR_A Character Map A bits 1:0.
1:0 CHAR_B Character Map B bits 1:0.
Table 6-52. Font Table
Code Font Table Location in Map 2 Code Font Table Location in Map 2
0 8 KB Block 0 4 8 KB Block 1
1 8 KB Block 2 5 8 KB Block 3
2 8 KB Block 4 6 8 KB Block 5
3 8 KB Block 6 7 8 KB Block 7
Index 04h
Typ e R /W
Reset Value 02h
VGA Memory Mode Register Bit Descriptions
Bit Name Description
7:4 RSVD Reserved.
3 CHAIN4 Chain4. When set to a 1, CPU address bits 1 and 0 are used to select the map or plane
in the frame buffer DWORD. For example, if CPU A1:A0 = 3, then map 3 is selected. If
CPU A1:A0 = 1, then map 1 is selected. If Chain4 is 0, then the frame buffer addressing
is controlled by the CHAIN2 (bit 2).
2 CHAIN2 Chain2. When set to a 0, CPU address bit 0 selects between frame buffer maps 0 and 1,
or maps 2 and 3, depending on the value in the graphics controller Read Map Select field
(Index 04h[1:0]). For example, if CPU A0 is 0, then map 0 (or 2) is selected.
1 EXT_MEM Extended Memory. This bit should always be set to a 1. It is a throwback to EGA where
the standard frame buffer size was 64 KB and was upgradeable to 256 KB. VGA always
has (at least) 256 KB.
0 RSVD Reserved.