Project Kiwi - an 68k Homebrew Computer

[psxphill]

Lovely, but why a 68008? Kamelito

1. less tracks on the board.
2. you can use one 8 bit rom to boot from rather than two or a 16 bit rom.
3. simpler ram interface logic.
 
I don't have a problem with 68008, but a couple of VIC IIs would have been more interesting.

[psxphill]

Just one question. You use a 68008 and say you have 4MB RAM. Since the 68008 only has 20 address bits, are you using bank switching for this?

The 68008 is available in 48 pin & 52 pin versions. The later has 22 address lines.
 
There is also the 68hc001, which is the replacement for the 68008 and can run in either 8 or 16 bit mode & has 24 address lines.

[psxphill]

So a byte wide bus means you need to use at least 2 extra cycles per instruction on a 68008 compared to a 68000. If it makes your machine easier to construct then by all means go for it, but it is like tieing one hand on your back.

I'm not sure. For most instructions the 68000 only accesses the bus every two cycles, where it fetches 2 bytes. I am too lazy to check, but it would not surprise me if the 68008 accesses the bus on every cycle. So it might not make much difference.

[psxphill]

That would be reasonable, but I always thought the 68008 was just an ordinary 68000 with only 8data lines exposed, thus is would also suffer the every other cycle memory access penalty.

It's not just a 68000 with 8 data lines, it has a0 as well.
 
My point about the speed was that on the 68000 it would read a word at cycle 0, cycle 2, cycle 4 etc. While the 68008 may read a byte on cycle 0, cycle 1, cycle 2, cycle 3 etc. So as long as you don't require access to the bus then it's the same.
 
If you did need to dma from memory as well as fetching then you could use 16 bit ram and buffer words and allow the second byte to be read from the buffer. However you're getting into being easier to just use a 68000. These days you could probably just use vram or double speed ram as well.
 
A 68SEC000 is probably the best bet & it can be clocked really fast, but the bus can also be configured as 8 bit or 16 bit. MOVE SR is privileged though, which makes it more like a 68010.