[Simh] C64 and C128

[Johnny Billquist]

On 2015-07-01 04:13, Bill Cunningham wrote:
>     ----- Original Message -----
>     *From:* Johnny Billquist <mailto:bqt at softjar.se>
>     *To:* simh at trailing-edge.com <mailto:simh at trailing-edge.com>
>     *Sent:* Tuesday, June 30, 2015 10:07 PM
>     *Subject:* Re: [Simh] C64 and C128
>
>     On 2015-07-01 01:56, Bill Cunningham wrote:
>      > I remember those floppy drives where big and heavy. I never had
>     cp/m or
>      > a c128. I am reading that an 8502 and Z80A (which I can't find
>     anything
>      > on) was inside. The Z80A was about 4 MHz. The Z80A word size I do not
>      > know. It was of course an 8 bit with a 16 bit address bus I
>     believe. Now
>      > which is "memory word" size?
>
>     You are asking very weird questions.
>     What do you mean by "word size"?
>
>     Johnny
>
>          I read in some specs either for 6502 or Z80A the term "memory
>     word size". These are all 8 bit cpus I know that but they have
>     different sized address buses. Rich straightened me out. The term
>     "memory" was throwing me off. I these cases Word Size would be 8
>     bit. Like todays 64 bit machines. Word size is now considered "64"
>     on 64 bit processors. Although the 6502 had 8 bit registers except
>     maybe for one, and a 16 bit address bus. They are 8 bit "word" sized.
>     Under control now. Thanks though.

But you are conflating many different things here.

If we start with something like the Z80, the memory address is 16 bits, 
while the data bus holds 8 bits. The basic CPU register size is 8 bits, 
but you have native instructions that also handle 16 bit data, and 
registers can be used in pairs, to have 16-bit CPU registers. And 
addresses are always expressed as 16-bit quantities.

The 6502 is similar to the Z80, but some addresses are sortof only 
expressed as 8-nit quantities, and I'm not sure you have any 16 bit 
registers in there that are for generic use, nor any native 16-bit 
instructions/operations.

A modern CPU is no easier. You might have 48 address bits on the 
physical address bus. 256 bits on the physical data bus. The virtual 
address is 64 bits, and the CPU have instructions that can work on 8, 
16, 32, 64 and possibly also larger data sizes. The registers normally 
holds 64 bits, though, and addresses are normally expressed as 64-bit.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol