h8300 sim: what is "eightbit" memory ?

duane@duaneellis.com duane@duaneellis.com
Wed Aug 17 18:35:00 GMT 2016


mike>> if eightbit is just an address mode, that's dirt simple to handle
mike>> a (void *)0 will go to the same memory location regardless of the

mike>> addr mode used by an insn. but it isn't what the sim is doing
today :(.

If I remember correctly, the arm simulator does various calculations and
pipeline tracking.
It's been a while, but it was doing this in the giant instruction switch
statement.

I've not read or looked at the H8 simulator does the H8 simulator sort
of "count instruction/clock time" - based on memory accesses?

The old 6502 had a very similar zero page feature (first 256 bytes) -
aka: a zero page instructions.

Accessing zero page using a zero page instruction required 3 clocks
(aka: the first 256 bytes)
Accessing absolute required 4 clocks, but you *CAN* access zero page in
absolute mode if you choose
This was useful when I wrote bit-bang uart simulation code (ie: Tight SW
loop - to emulate a uart via GPIO pin)

Thus - from a logical view the "memory is just memory" end of story.

if this is not evident in the simulation code, this might have been a
planned feature that never materialized or was never finished.

this wiki page talks about it:

https://en.wikipedia.org/wiki/Zero_page





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