Commonly, to perform something like an arbitrary width addition, these CPUs have a carry flag that is set if an operation overloads, so that the carry can be ad

Commonly, to perform something like an arbitrary width addition, these CPUs have a carry flag that is set if an operation overloads, so that the carry can be added into the next addition. This way you can simply chain simple 8-bit additions to get the desired operation width. For example, a 16-bit addition (0x5555 + 0x1234 on 6502 clc lda a adc b sta res lda a+1 adc b+1 sta res+1 a: .byte $55, $55 b: .byte $34, $12 res: .byte $00, $00 ; reserved for result