in reply to What was your first program?

Here is an example of some of the first code I wrote.

Can you tell what each does and what kind of computer it might rub on.

Mystery Code 1: 

start    LDY #$FF    
loop1    LDX #$FF  
loop2    DEX       
    BNE loop2  
    DEY        
    BNE loop1  
    RTS

[download]
Mystery code 2: 
mlt16   LDA #$00     
        STA $26
        STA $27
        LDX #$16     
nxtbt   LSR $21      
        ROR $20
        BCC align    
        LDA $26      
        CLC
        ADC $22      
        STA $26      
        LDA $27      
        ADC $23      
align   ROR A        
        STA $27      
        ROR $26
        ROR $25
        ROR $24
        DEX          
        BNE nxtbt    
        RTS

[download]

The first correct answer to both questions will be sent a genuine picture/jpeg of an orginal single-sided, single-density 8 inch floopy disk drive (I think maybe). mitd-Made in the Dark
'My favourite colour appears to be grey.'

Replies are listed 'Best First'.
(jcwren) RE: (2) What was your first program?
by jcwren (Prior) on Nov 14, 2000 at 19:19 UTC
    Both run on 6502 processors.

    Code #1 is a delay loop.
    Code #2 is a 16 bit unsigned multiply (multiplies $20/$21 by $22/$23, returning the result in $24/$25/$26/$27).

    Update: OK, I couldn't resist. I dug out the 6502 assembler and emulator to test code #2. Now, admittedly, the emulator could be broken, but I didn't get any results that made sense for either a multiply or divide. I spent about 45 minutes playing with this, and the simulator seems to be executing as I would expect. But the resulting $26/$27 locations never come up with any valid values using $20/$21 and $22/$23 as inputs (the higher address is the high byte of each word, so I had byte ordering correct).

    And since code #1 was going for maximum delay, you could have gotten a little more delay by changing it to: 
    start	LDY #$FF    
loop1	LDX #$FF  
loop2	DEX       
	BNE loop2  
	DEY        
	BNE loop2  
	RTS
    You're resetting X to #$FF, when it's already 0, losing 255 free 2 cycle delays.

    --Chris

    e-mail jcwren
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