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The 6502 processor has six registers. They are 8-bit, with the exception of the Program Counter, which is 16-bit.
Accumulator(A) - The accumulator can read and write to memory. It is used for arithmetic and logic.
X Index - The x index is can read and write to memory. It is used primarily as a counter in loops, or for addressing memory, but can also temporarily store data like the accumulator.
Y Index - Much like the x index, however they are not completely interchangeable. Some operations are only available for each register.
Flag(P) - Seven bits that represent the status of the processor.
Stack Pointer(S) - The stack pointer hold the address to the current location on the stack. The stack is a way to store data by pushing or popping data to and from a section of memory.
Program Counter(PC) - This register keeps up with the processor's current location in a program.
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To multiply the value in A by two, use the instruction ASL A.
To divide the value in A by two, use the instruction LSR A.
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The following routine multiplies two unsigned 16-bit numbers, and returns an unsigned 32-bit value.
mulplr = $c0 ; ZP location = $c0 partial = mulplr+2 ; ZP location = $c2 mulcnd = partial+2 ; ZP location = $c4 _usmul: pha tya pha _usmul_1: ldy #$10 ; Setup for 16-bit multiply _usmul_2: lda mulplr ; Is low order bit set? lsr a bcc _usmul_4 clc ; Low order bit set -- add mulcnd to partial product lda partial adc mulcnd sta partial lda partial+1 adc mulcnd+1 sta partial+1 ; ; Shift result into mulplr and get the next bit of the multiplier into the low order bit of mulplr. ; _usmul_4: ror partial+1 ror partial ror mulplr+1 ror mulplr dey bne _usmul_2 pla tay pla rts
Here's an example of the above _usmul routine in action, which multiplies 340*268:
lda #<340 ; Low byte of 16-bit decimal value 340 (value: $54) sta mulplr lda #>340 ; High byte of 16-bit decimal value 340 (value: $01) (makes $0154) sta mulplr+1 lda #<268 ; Low byte of 16-bit decimal value 268 (value: $0C) sta mulcnd lda #>268 ; High byte of 16-bit decimal value 268 (value: $01) (makes $010C) sta mulcnd+1 lda #0 ; Must be set to zero (0)! sta partial sta partial+1 jsr _usmul ; Perform multiplication ; ; RESULTS ; mulplr = Low byte of lower word (bits 0 through 7) ; mulplr+1 = High byte of lower word (bits 8 through 15) ; partial = Low byte of upper word (bits 16 through 23) ; partial+1 = High byte of upper word (bits 24 through 31) ;
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If you keep score in a binary number, you must convert it to a sequence of digits before displaying it. The article 16-bit BCD lists a subroutine to do this.
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Since the NES can't easily do something like printf()
(or echo
for those familiar with scripting), one of the easiest ways to test code is to output some audio. Something along the lines of...
reset: lda #$01 ; square 1 sta $4015 lda #$08 ; period low sta $4002 lda #$02 ; period high sta $4003 lda #$bf ; volume sta $4000 forever: jmp forever