Re: question about CPU operations

[toddpw@cco.caltech.edu (Todd P. Whitesel)]

dockery@griffin.cuc.ab.CA (Sean Dockery) writes:

>In the Apple II Reference Manual, it states that the 1.023MHz 6502
>requires approximately 300ns for a read operation and 100ns for a
>write operation.

This is true of the chip itself. However, there are requirements of the other
parts (memory, etc.) that affect the answer to the real question you're asking.

>However, it is also documentation that performing a write operation is
>carried out by first performing a read of the datum in a memory
>location followed by a write of the datum in a given register.

Not all writes happen like that. I'd have to look things up to be sure, but I
believe it is that only indexed writes do that.

>	Does the write operation of the 6502 CPU require 400ns or the
>	100ns as stated?  I have a feeling that a write *operation*
>	actually requires but 100ns, but a write *instruction*
>	requires 400ns to execute.

You're getting really warm. A write *cycle* takes 978ns just like a read cycle,
but the actual reading and writing takes different amounts of time depending
on which part of the system is doing it. A write *instruction* takes from 3 to
5 (6 maybe?) cycles depending on which addressing mode you use, and each of
those cycles takes 978ns (except in the case of the "long" cycle; these happen
at the end of every video scan line).

>Can anyone clarify which is the case?  How long does it actually take?

As far as the CPU is concerned, the actual write takes 100ns. But there's other
stuff that has to happen, and so it really takes more like 700ns or so; the CPU
has to feed out an address and the memory has to interpret and prepare to take
the data, before actually getting it. Since there is also overhead in the CPU
switching to the next address and read/write operation, the overall cycle time
is usually 978ns and this is enforced by the master timing generator.

This gets really complicated if you want to dig in any deeper -- I heartily
recommend Understanding The Apple II by Jim Sather, and The Apple II Circuit
Description. Both of these are great books for making sense of the intricate
elegance that is the Apple II.

Todd Whitesel
toddpw @ cco.caltech.edu