Re:8-bit computer from scratch
by TheOtherHobbes on Saturday June 04, 2016 @09:20PM
(#11839167)
One of the features of the Z80 was built-in memory refresh, so it needed a lot less glue logic than most processors, which had to have external registers and latches to handle the refresh cycles.
Way back I used to hand solder Z80 embedded controller board prototypes, and they never needed much in the way of chippery.
In an S100 system the biggest extras were the video (formerly VDU) cards, and the memory cards.
Both were limited by the RAM of the day, because RAM came in tiny sizes like 1 bit x 16k dynamic or 2k x 8 static. So you needed a lot of chips on a separate board for what was really not much RAM at all.
Also, money. 64k boards cost over $1000, which is > $3k in modern money.
Now you can get 8 bit x 256k static RAM on a single chip, so you can park that next to the processor without a separate card. With a bit of glue you can even share it with the graphics hardware, eliminating any need for separate graphics RAM.
It's interesting to see this done with so few ICs. Early Z80 machines had a much higher part count, because a lot of external support was needed to talk to anything. Today we have the "system on a chip", with lots of onboard peripherals, but a Z80 is just a CPU.
The predecessor to the "system on a chip" was the semi-custom "junk chip", which contained unrelated stuff the standard chips didn't have. He may be using some of those to get the parts count down. Can't tell; schematic too small.