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What am I doing wrong?

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65024ever

Post subject: What am I doing wrong?

Posted: Sat May 29, 2010 2:02 am

Joined: Mon Apr 12, 2010 8:03 pm
Posts: 1

Hi everyone, I'm relatively new to assembly coding in general, and as a little exercise, I wanted to port the brainf*** language to the 6502 (on the apple 1)

This is the first module of the code, assembles with KRUSADER.
What it's supposed to do is pull an ascii byte off the keyboard, store it in the first buffer, then check to see if it's one of the 8 commands. If so, it also stores it in the second buffer. I don't understand, but it's not working right at all. do I have to have BNE instructions for every CMP?

Code:

000 KBD    .=  $D010
001 BUFF   .=  $300
002 ECHO   .=  $FFEF
003 KBDRDY .=  $D011
004 BUFF2  .=  $400
005 START  .M  $500
006        LDY #$00
007        LDX #$00
008        CLD 
009 GET    LDA KBDRDY
00A        BPL GET
00B        LDA KBD
00C        JSR ECHO
00D        STA BUFF,X
00E        INX 
00F COMP   CMP #$3E
010        BEQ STR
011        CMP #$3C
012        BEQ STR
013        CMP #$2B
014        BEQ STR
015        CMP #$2D
016        BEQ STR
017        CMP #$2E
018        BEQ STR
019        CMP #$2C
01A        BEQ STR
01B        CMP #$5B
01C        BEQ STR
01D        CMP #$5D
01E        BEQ STR
01F        CMP #$1B
020        BEQ PARSE
021        JMP GET
022 STR    STA BUFF2,Y
023        JMP GET
024 PARSE  JMP $FF1F

FFIF is the re entry point of the monitor.

Thanks for the help!

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8BIT

Post subject:

Posted: Sat May 29, 2010 4:17 am

Joined: Fri Aug 30, 2002 9:02 pm
Posts: 1748
Location: Sacramento, CA

After line 22, you need to increment the Y register (INY).

Daryl

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BigDumbDinosaur

Post subject:

Posted: Tue Jun 01, 2010 4:34 am

Joined: Thu May 28, 2009 9:46 pm
Posts: 8514
Location: Midwestern USA

8BIT wrote:

After line 22, you need to increment the Y register (INY).

Daryl

Correct. Also, does the ECHO subroutine preserve the registers?

_________________
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Nightmaretony

Post subject:

Posted: Wed Jun 02, 2010 5:10 pm

Joined: Fri Jun 27, 2003 8:12 am
Posts: 618
Location: Meadowbrook

Might be more feasible to rewrite into a checking loop that checks against a data table instead of doing individual compare and branches.

1. makes the code smaller.
2. If your table gets larger, you run up against the 128 byte max jump on the BEQ command, forcing you to use a jump table (I found that out on my pinball program)
3. You can add more commands to the table and change the end size much more easily than keeping on adding code.

Code:

CommandTable
    .DB $aa, $bb, $cc, $dd, $ee, $ff, $00 (end byte is zero marker)
TablePointer = #$00
     ldy TablePointer

LOOP
     jsr   GetKey     ; returns key in KeyPressed memloc)
     lda   CommandTable, TablePointer
     cmp KeyPressed
     beq ThisIsACommand
     cmp #$00        ; end of table?
     beq NoCommandfound
     iny
     jmp LOOP

and this will leave the command in the KeyPressed and Accumulator locations.

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