The I/O Register block contains 3 bytes of PC-compatible I/O registers (8255 PPI). These registers are defined as follows:
PORT BIT(S) MODE DESCRIPTION
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60h 0-7 R/W If port 61h bit7 = 0: Scratch location for keyboard scan code
See "Keyboard Controller" on page 15-543 for more information.
0-7 R/W If port 61h bit = 1: Scratch location for SW1 configuration
switch settings
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61h 0 R/W TIMER 2 gate
1 R/W PC-compatible speaker data
2 R/W Ignored, always reads 0
3 R/W Select source/destination for port 62h bits 0-3
4-5 R/W Ignored, always read 0
6 R/W 0 = disable keyboard
7 R/W Select source/destination for port 60h
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62h 0-3 R/W If port 61 bit 3 = 0: Scratch location for 4 MSBs of SW2
switch settings
0-3 R/W If port 61 bit 3 = 1: Scratch location for 4 LSBs of SW2
switch settings
4 R Unused (reads 0)
5 R TIMER 2 output
6-7 R Unused (read 0)
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