CODEWITHSUNDEEP
assembly6502
jak_es
jak_es

Reputation: 19

256 and 64 bit number in assembly 6502

I have to Write a code in 6502 assembly language that divides 256-bit numbers by 64-bit numbers using basic bit shift operations, and arithmetic and logic operations. i just don't know how to write 256 and 64 bit number in assembly 6502.

Upvotes: 1

Views: 643

Answers (1)

Sep Roland
Sep Roland

Reputation: 39166

i just don't know how to write 256 and 64 bit number in assembly 6502.

You would store these as strings of 32 and 8 bytes respectively.

In memory, the qword number $1122334455667788 would look like these 8 bytes:

$88, $77, $66, $55, $44, $33, $22, $11

Similar and 4 times longer for a 256-bit number.

To get an idea of how to work with multibyte numbers. This is the addition of a couple of 16-bit numbers.
If NumA is $1122 and NumB is $3344, then the sum will be Res:

CLD
CLC
LDA NumA    ; -> A = $22
ADC NumB    ; -> A = $22 + $44 = $66
STA Res
; The carry propagates to the higher order addition
LDA NumA+1  ; -> A = $11
ADC NumB+1  ; -> A = $11 + $33 = $44
STA Res+1

Res now holds the sum $4466.

Working with very big numbers will require a loop. Next is the addition of two qwords:

 CLD
 CLC
 LDY #8        ; Qwords have 8 bytes
 LDX #0
Loop:
 LDA NumA,X
 ADC NumB,X
 STA Res,X
 INX           ; INX and DEY don't clobber the carry
 DEY           ; and thus it can propagate
 BNE Loop

Upvotes: 3

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