9 Feb
2003
9 Feb
'03
12:53 a.m.
On Feb 8, 19:02, Eric Smith wrote:
Well, I haven't counted them, but pretty much all
instructions that
use the ALU do that. This includes ADC, SBC, ORA, AND, EOR, BIT,
and (when the accumulator is the destination) LSL, ASR, ROR, and ROL.
And the PLP and PLA instructions, which increment the stack pointer
using the ALU during the next fetch.
Are you sure? My understanding is that PLP and PLA increment the stack
pointer *before* fetching the byte off the stack. The 6502 stack
pointer always points to the next free location on the stack. It's
possible that it does something like:
fetch instruction
decode, and set up ALU to generate SP+1
fetch [SP+1]
add 1 to SP while fetching next instruction
Maybe you're thinking of PHP and PHA ([SP]:=A; SP:=SP+1), which take
one cycle less than PLP/PLA?
--
Pete Peter Turnbull
Network Manager
University of York