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Technical Reference Guide
Configuration Byte 26h, Auxiliary Peripheral Configuration
Default Value = 00h
Bit
Function
7,6
I/O Delay Select
00 = 420 ns (default)
01 = 300 ns
10 = 2600 ns
11 = 540 ns
5
Alternative A20 Switching
0 = Disable port 92 mode
1 = Enable port 92 mode
4
Bi-directional Print Port Mode
0 = Disabled
1 = Enabled
3
Graphics Type
0 = Color
1 = Monochrome
2
Hard Drive Primary/Secondary Address Select:
0 = Primary
1 = Secondary
1
Diskette I/O Port
0 = Primary
1 = Secondary
0
Diskette I/O Port Enable
0 = Primary
1 = Secondary
Configuration Byte 27h, Speed Control/External Drive
Default Value = 00h
Bit
Function
7
Boot Speed
0 = Max MHz
1 = Fast speed
6..0
Reserved
Configuration Byte 28h, Expanded and Base Memory, IRQ12 Select
Default Value = 00h
Bit
Function
7
IRQ12 Select
0 = Mouse
1 = Expansion bus
6,5
Base Memory Size:
00 = 640 KB
01 = 512 KB
10 = 256 KB
11 = Invalid
4..0
Internal Compaq Memory:
00000 = None
00001 = 512 KB
00010 = 1 MB
00011 = 1.5 MB
.
.
11111 = 15.5 MB
Compaq Deskpro and Evo Personal Computers4-29
Featuring Intel Celeron and Pentium III Processors
Fifth Edition - March 2002