Two Stop Bits

A detailed technical analysis of the ZX81 keyboard scanning subroutine from the ROM at address 0x02BB. The routine uses an 8-iteration loop to scan an 8×5 keyboard matrix, encoding keypresses into HL registers where bits correspond to horizontal and vertical zones. The commentary walks through the Z80 assembly code instruction-by-instruction, explaining how the routine handles the shift key specially, uses bit manipulation tricks like SBC A,A for conditional logic, and interfaces with the keyboard hardware through I/O ports. Includes keyboard zone diagrams, hardware schematics, a Z80 assembly cheatsheet, and a BASIC program to inspect the scan codes.

ZX81 KEYBOARD Subroutine—Commentary