Over in
this post I included an old and not very efficient subroutine I wrote for converting an ASCII digit into the equivalent binary value. Today, just for kicks, I asked ChatGPT it it could analyze 6502 machine code, and gave it that subroutine to chew on.
ChatGPT wrote:
This code appears to be a routine that converts an ASCII character representing a hexadecimal digit into its corresponding numeric value. Here is how the code works:
1. The routine begins by pushing the processor status register onto the stack (phy).
2. The LDY #$00 instruction initializes the Y register to zero, which will be used as an index into the HEX_TABLE lookup table.
3. The .next_upper label begins a loop that compares the value in the HEX_TABLE table at the index specified by the Y register to the character being converted. If a match is found, the code jumps to the .found label.
4. If no match is found, the INY instruction increments the Y register, and the CPY #$10 instruction checks whether the Y register has reached the end of the table. If it has, the code jumps to the .try_lower label.
5. If the Y register has not reached the end of the table, the code branches back to the .next_upper label to continue searching.
6. The .try_lower label initializes the Y register to 10 (the index of the first lowercase hexadecimal digit in the HEX_TABLE table).
7. The .next_lower label begins a loop that compares the value in the HEX_TABLE table at the index specified by the Y register to the character being converted. If a match is found, the code jumps to the .found label.
8. If no match is found, the CPY #$10 instruction checks whether the Y register has reached the end of the table. If it has, the code jumps to the .not_digit label.
9. If the Y register has not reached the end of the table, the code branches back to the .next_lower label to continue searching.
10. The .not_digit label restores the processor status register from the stack (PLY), sets the accumulator to #$FF (indicating that the character is not a valid hexadecimal digit), and returns from the routine (RTS).
11. If a match is found, the TYA instruction copies the index in the Y register (corresponding to the numeric value of the hexadecimal digit) to the accumulator. The processor status register is restored from the stack (PLY), and the routine returns with the result in the accumulator (RTS).
As for how to improve this code, that depends on what your specific goals are for the routine. Are you trying to optimize for speed, minimize code size, or improve readability? Are there any specific issues or limitations you're running into?
I was fairly impressed by this level of understanding, especially given that in our previous conversation about violin etudes it consistently was confused about technical specifics, and at one point told me about an etude book that it made up on the spot. (It was a bit like talking to Borges...). Here there are a few little errors, but it is mostly right!
So I asked it: Can we both improve speed and shrink code size?
Its reply was accurate, but disappointingly general (Optimize loops, use ZP addressing, etc.). But then I asked it to write its own version:
"Can you show me some 6502 machine language code that will convert an ASCII character representing a hexadecimal digit into its corresponding numeric value? It should return $FF if the input is not an ASCII character representing a hexadecimal digit."
ChatGPT wrote:
Certainly! Here's an example implementation of such a function:
Code:
; Input: A = ASCII character to convert
; Output: A = corresponding hexadecimal digit value, or $FF if not valid
charToDigit:
cmp #$30 ; Is input < "0"?
bmi .not_hex_digit
cmp #$3a ; Is input >= "a"?
bcc .found
cmp #$41 ; Is input >= "A"?
bcc .not_hex_digit
cmp #$47 ; Is input <= "F"?
bcs .not_hex_digit
sec
sbc #$30 ; Convert from ASCII to numeric value
rts
.not_hex_digit:
lda #$ff ; Return $FF for invalid input
rts
.found:
sec
sbc #$37 ; Convert from ASCII to numeric value for letters
rts
You can call this function with the input character in the A register and it will return the corresponding hexadecimal digit value in A, or $FF if the input is not a valid hexadecimal digit.
It's a good try! I asked it some leading questions to see if I could get it to produce one that would actually work, but the tries got progressively worse. The second try used a "bhi" instruction, and the third one was basically gibberish.
This seems to be a theme with ChatGPT. It's pretty good with concepts, but falls down on specific technical details.
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