IBM PS/1 mod.2011 POS registers

Port hex 0094
bit 5 VGA setup mode
bit 7 Board setup mode


System Board POS Register 2 (Hex 102)
-------------------------------------

VGA
bit 0
(Tech Ref. 3-24 p.94)
When in Setup mode (l/O address hex 0094, bit 5 equals 0), the
VGA responds to a single option select byte at l/O address hex
0102 and treats the LSB (bit 0) of that byte as the VGA sleep bit.
When the LSB is 0, the VGA does not respond to commands,
addresses, or data on the data bus. When the LSB is 1, the VGA
responds. If the VGA was set up and is generating video output
when the LSB is set to 0, the output is still generated.

The VGA responds only to address hex 0102 when in the Setup
 mode. No other addresses are valid at that time. Conversely, the
VGA ignores address hex 0102 when in the Enabled mode (I/O
address hex 0094, bit 5 equals 1), and decodes normal I/O and
memory addresses.


Parallel Port 
bit 7
bit 6
bit 5
(Tech Ref. 3-150 p.220)
The parallel port can be configured to three different address spaces
previously used in IBM Personal Computer products. These
addresses are selected by placing the system board in Setup; clear
bit 7 of port hex 94. Perform an I/O write to port hex 0102. Bits 6 and
5 in port hex 0102 are used to select the address spaces shown in
Figure

When setup is complete, be sure to set bit 7 of port hex 94.

Bit 6   Bit 5   Function
0       0       Parallel 1 LPT1:
0       1       Parallel 2 LPT2:
1       0       Parallel 3 LPT3:

The Extended mode option of the parallel port is selected through
Programmable Option Select. With the system board in Setup, the
Extended mode is selected by writing a O to bit 7 of I/O address hex
0102.

bit 4: 1=parallel port enabled

COM serial port
Bit 2 is cleared at F000:204B (disable serial port?)
Bit 0,2,3 are checked at POST 64 (F000:1A0D)

bit 2: 1=serial port enabled
bit 3: COM port address. 1=COM1 (port 3F8h, irq 4), 0=COM2 (port 2F8h, irq 3) 
       fixed to 1 on model 2011

Other
Bit 0 set to 1 before setting POS 3 Bit 3. (F000:4CE3)

Bit 0,4,7 set to 1, others to 0, at POST_START

Bit 3 always 1 (F000:026D)

System Board POS Register 3 (Hex 103)
-------------------------------------
F000:0247
F000:0FC7
F000:1361
bit3 set to 1 before reading port 320 to check if HDD adapter responds (F000:4684)
bit3 set to 0 if port 91h returns 0 after reading port 320. (F000:45A5)


System Board POS Register 4 (Hex 104)
-------------------------------------
(Tech Ref. 3-157 p.227)
With the system board in setup, the system board memory can
be disabled by writing port address hex 104 as shown in the
following figure.

Bits  Function                  Hex Range
7-5   Reserved
4     Enable Fifth 128KB Bank   080000 - 09FFFF
3     Enable Fourth 128KB Bank  060000 - 07FFFF
2     Enable Third 128KB Bank   040000 - 05FFFF
1     Enable Second 128KB Bank  020000 - 03FFFF
0     Enable First 128KB Bank   000000 - O1FFFF


System Board POS Register 5 (Hex 105)
-------------------------------------
Bits
3	memory timing/wait states?

At boot bit 3 is 1. If memory test fails then POST set bit 3 to zero and repeats from POST 0Eh.
If memory test fails again then POST emits the appropriate error code.

F000:0552
F000:05CF




