Linked by Thom Holwerda on Sat 7th Sep 2013 09:54 UTC
Hardware, Embedded Systems

The 8-bit Z-80 processor is famed for use in many early personal computers such the Osborne 1, TRS-80, and Sinclair ZX Spectrum, and it is still used in embedded systems and TI graphing calculators. I had always assumed that the ALU (arithmetic-logic unit) in the Z-80 was 8 bits wide, like just about every other 8-bit processor. But while reverse-engineering the Z-80, I was shocked to discover the ALU is only 4 bits wide! The founders of Zilog mentioned the 4-bit ALU in a very interesting discussion at the Computer History Museum, so it's not exactly a secret, but it's not well-known either.

I have been reverse-engineering the Z-80 processor using images from the Visual 6502 team. The image below shows the overall structure of the Z-80 chip and the location of the ALU. The remainder of this article dives into the details of the ALU: its architecture, how it works, and exactly how it is implemented.

Ken Shirrif's blog is an absolute must for fans of ultra-low-level hardware stuff. This goes way over my head, but interesting nonetheless.

Thread beginning with comment 571623
To view parent comment, click here.
To read all comments associated with this story, please click here.
Member since:

No, plenty of cpus have used serial computation and had reasonable performance, TI 9900 did 16 bits in 18 clocks or so, it allowed the clock to run faster to make up for it. The architecture defines the width of a processor, not the internal design. And the Pentium 4 also used a 16 bit ALU double pumped, still a 32b processor.

Reply Parent Score: 3

puenktchen Member since:

The architecture defines the width of a processor, not the internal design.

I alway thought the internal design was part of the architecture. And that the meaning of 8/16/32/64 bitness changed over the years with a little help by marketing. So the width of the processor is defined by the instruction set, not by the data path or registers or .. ?

Reply Parent Score: 3

Drumhellar Member since:

It's defined by the instruction set, and not the actual implementation

Z80 is an 8-bit architecture because you add two 8-bit registers to get a result. The fact that behind the scenes, it's breaking it down into multiple 4-bit adds is inconsequential. They could change it at a later point to give it a true 8-bit ALU and nobody would know the difference.

Same about data buses. The 8086 had a 16-bit system bus, while the 8088 had an 8-bit bus. This wouldn't make the 8088 an 8-bit chip, since the you were doing 16-bit math in 16-bit registers.

But, there was time when it was reasonable to assume that 8-bit chips had 8-bit buses and 16-bit chips had 16-bit buses, but as time progressed the ISA became further and further divorced from the actual implementation.

Reply Parent Score: 4