This is an old revision of the document!
Table of Contents
The 6821 PIA
This is a generic I/O chip designed by Motorola and manufactured by Thomson as a second source. It is present in every Thomson machine.
Programming and architecture
Uses
MO "System" PIA
data port A
- bit 0: /FORME - Switch the screen RAM mapping between pixel and attribute RAMs
- bits 1-4: border color (R,G,B,P)
- bit 5: light pen button
- bit 6: tape drive data output
- bit 7: tape drive data input
Bit 7 is low when no tape drive is plugged, and high when there is one. The monitor loading and saving code checks for this to detect the ape drive.
data port B
- bit 0: sound output
- bits 1-3: keyboard column to scan
- bits 4-6: keyboard line to scan
- bit 7: state of key selected by the column and line
