An actual multiply instruction in 8bit?

[Dio]

Actually, that's a great point: the 6809 was pretty close to a third-generation 8-bit CPU and it's got a lot more horsepower than the others.

[Harlequin]

Actually, that's a great point: the 6809 was pretty close to a third-generation 8-bit CPU and it's got a lot more horsepower than the others.

Was it not creeping into the area, though, of wobbly definitions of 8- and 16-bit?

[Dio]

Not really. 8-bit data bus, 8-bit ALU, mostly 8-bit register organisation internally.

[gilesgoat]

Actually, that's a great point: the 6809 was pretty close to a third-generation 8-bit CPU and it's got a lot more horsepower than the others.

It is still "random logic" or does it already use some for of microcode/bit-slicer architecture ?

I have some interesting OLD books about "Microprocessors Design via Bit-Slicers" .. you DIO SHOULD know "something" as it's all about those AM290X "bit slicer chips" that normally are "sequencers" and other things ..

Yeah I think 6809 was quite evolved, but I never worked on it.

[Dio]

Well, all the microprocessors used some sort of ROM to do the first round decode, which if you squint hard enough you might call microcode, but all the 8-bits used random logic sequencing as I understand it. A microcode sequencer is really big. The 68000 compressed theirs and it was still 3-4 times bigger than the entire ARM1 design.

The bit-slice thing was all about the early 70s minicomputers and mainframes. Until the mid-late 70s it wasn't regarded as possible to build a high capability single-chip 16-bit or larger computer (although frankly I think it probably was possible to manufacture an ARM1-class chip in 1975) so there were these various SSI 4-bit ALU chips and chips of 'registers' and the like that you used to build the larger machine. Ken Shirriff has a bunch of stuff on the Alto, which was very much that kind of design, if you want to dig deeper.

[skab]

6809 does have MUL of two 8-bit registers to a 16 bit result and takes 10/11 cycles.

It did, didn't it? I was about to ask when I read that comment that none of the 8-bits had multiplication instructions. So that one did... But then it came really late in the game, the 68000 was already lurking the background when the 6809 came out.

[tufty]

WRT divides,

https://homepage.divms.uiowa.edu/~jones/bcd/divide.html