Version analysis of ZEKTRAN - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

6502 Second Processor and Master

Code variations between these versions are shown below.

       Name: ZEKTRAN
       Type: Subroutine
   Category: Keyboard

Code variation 1 of 6A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Master
6502SP
    Summary: Clear the key logger
    Summary: Reset the key logger buffer at KTRAN

Code variation 2 of 6A variation in the comments only

This variation is blank in the 6502 Second Processor version.

Master
 Returns:

   X                    X is set to 0


.ZEKTRAN

Code variation 3 of 6Minor and very low-impact

This variation is blank in the Master version.

6502SP
 LDX #11                \ We use the first 12 bytes of the key logger buffer at
                        \ KTRAN, so set a loop counter accordingly

 LDA #0                 \ We want to zero the key logger buffer, so set A % 0

Code variation 4 of 6Minor and very low-impact

This variation is blank in the 6502 Second Processor version.

Master
 LDX #17                \ We want to clear the 17 key logger locations from
                        \ KL to KY20, so set a counter in X

.ZEKLOOP

Code variation 5 of 6Minor and very low-impact

Tap on a block to expand it, and tap it again to revert.

Master
6502SP
 STA JSTY,X             \ Store 0 in the X-th byte of the key logger

 DEX                    \ Decrement the counter

 BNE ZEKLOOP            \ And loop back for the next key, until we have just
                        \ KL+1. We don't want to clear the first key logger
                        \ location at KL, as the keyboard table at KYTB starts
                        \ with offset 1, not 0, so KL is not technically part of
                        \ the key logger (it's actually used for logging keys
                        \ that don't appear in the keyboard table, and which
                        \ therefore don't use the key logger)
 STA KTRAN,X            \ Reset the X-th byte of the key logger buffer to 0

 DEX                    \ Decrement the loop counter

 BPL ZEKLOOP            \ Loop back until we have zeroed bytes #11 through #0

 RTS                    \ Return from the subroutine

Code variation 6 of 6Minor and very low-impact

This variation is blank in the Master version.

6502SP
 RTS                    \ This instruction has no effect as we already returned
                        \ from the subroutine