Answer
The Ex__4_checksum_alg.asm
file contains the following checksum code:
; Ex__4_checksum_alg.asm ; Try running this code at https://skilldrick.github.io/easy6502/ ; Set up the array of bytes to be checksummed LDA #$01 STA $00 LDA #$72 STA $01 LDA #$93 STA $02 LDA #$F4 STA $03 LDA #$06 ; This is the checksum byte STA $04 ; Store the address of the data array in $10-$11 LDA #$00 STA $10 STA $11 ; Store the number of bytes in X LDX #5 ; Entering the checksum algorithm ; Move X to Y TXA TAY ; Compute the checksum LDA #$00 DEY LOOP: CLC ADC ($10), Y DEY BPL LOOP CMP #$00 BNE ERROR ; The sum is zero: Checksum is correct LDA #1 JMP DONE ; The sum is nonzero: Checksum is incorrect ERROR: LDA #0 ; A contains 1 if checksum is correct, 0 if it is incorrect DONE: