Why is LDA/STA Absolute,X as fast ZeroPage,X?

[darkatx]

According to my book here - normal LDA absolute uses 4 cycles where as Zero-page is only 3 cycles.
Yet, when they're indexed they both are listed as executing in 4 cycles - is this correct? I'm using the CBM debugger and it's confirming that this is the case.

[groepaz]

what you are seeing there is an optimization.... if (and only if) the result of adding X to the low byte of the address does NOT result in a carry over to the high byte, then the 5th cycle can be omitted. consequently if that is the case, then you will observe lda abs,x actually takes those 5 cycles. (your book should mention "penalty cycles" or something alike ... if not, get another book =))

[darkatx]

It's the machine language book by Butterfield...so I must have missed something there on the chart. LOL...thanks man

[Mike]

The 65xx can add the X-register to the low-byte of the address while the high-byte is fetched over the bus.

If then there is a carry with LDA, the CPU will first do a read access from a wrong address (i.e. with the uncorrected high-byte), say "oops", and then re-do the fetch with the correct address.

That's the reason a STA ABS,X always takes 5 cycles: the CPU can't afford to trash memory. For this reason, the first access 'attempt' is converted to a read access (to the possibly wrong address [1]) whose result is discarded, and then the second access is the write to the correct address.

These spurios read accesses can make problems with read sensitive I/O registers, so you have been warned.


[1]: on CMOS variants, the address behind the current instruction is read instead

[darkatx]

I've reached a point where I'm getting really interested in what's going on at the Phase 1 and Phase 2 level. I appreciate the warning about the read sensitive I/O registers going hanky.
Got some more reading to do - thanks man

[groepaz]

to examine that kind of details, the simulator from visual6502.org can be really helpful (as it shows every cycle with everything you ever wanted to know - and more )

[darkatx]

Hey thanks for the link man!