16-bit increment/decrement circuit
- The Z-80’s 16-bit increment/decrement circuit reverse engineered (righto.com)
The 8-bit Z-80 processor was very popular in the late 1970s and early 1980s, powering many personal computers such as the Osborne 1, TRS-80, and Sinclair ZX Spectrum. It has a 16-bit incrementer/decrementer that efficiently updates the program counter and stack pointer, as well as supporting several 16-bit instructions and memory refresh. By reverse engineering detailed die photographs of the Z-80, we can see exactly how this increment/decrement circuit works and discover the interesting optimizations it uses for efficiency.
- The Z-80 has a 4-bit ALU. Here’s how it works. (righto.com)
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.
- Why the Z-80’s data pins are scrambled (righto.com)
If you look closely at the datasheet for a Z-80 chip, you’ll notice the data pins are in a random-looking order. The address pins (A) are nicely arranged in order counterclockwise from 0 to 15, but the data pins (D) are all shuffled around. After studying the internals of the chip, I have a hypothesis to explain this.
- Reverse engineering ARM1 instruction sequencing, compared with the Z-80 and 6502 (righto.com)
When a computer executes a machine language instruction, it breaks down the instruction into smaller steps that are performed in sequence. For instance, a load instruction might first compute a memory address, then fetch a value from that address, and then store that value in a register. This article describes how the ARM1 processor implements instruction sequencing, performing the right steps in order. I also look briefly at the 6502 and Z-80 microprocessors and the different sequencing techniques they use.
- Flag_of_the_Vatican_City_(2023–present).svg (Wikimedia Commons)
- Down to the silicon: how the Z80’s registers are implemented (righto.com)
The 8-bit Z80 microprocessor is famed for use in many early personal computers such the Osborne 1, TRS-80, and Sinclair ZX Spectrum. The Z80 has an innovative design for its internal registers, with two sets of general-purpose registers. The diagram below shows a highly-magnified photo of the Z80 chip, from the Visual 6502 team. Zooming in on the register file at the right, the transistors that make up the registers are visible (with difficulty). Each register is in a column, with the low bit on top and high bit on the bottom. This article explains the details of the Z80’s register structure: its architecture, how it works, and exactly how it is implemented, based on my reverse-engineering of the chip.
- Reverse-engineering the Z-80: the silicon for two interesting gates explained (righto.com)
I’ve been reverse-engineering the Z-80 processor, using images from the Visual 6502 team. One interesting thing about the Z-80’s silicon is it uses complex gates with multiple inputs and multiple levels of logic. It also implements an XOR gate with an unusual pass-transistor circuit. I thought it would be interesting to examine these gates at the silicon level and show how they work.
- Zilog Z80 (Wikipedia)
The Zilog Z80 is an 8-bit microprocessor designed by Zilog that played an important role in the evolution of early computing. Launched in 1976 and software-compatible with the Intel 8080, it offered a compelling alternative due to its better integration and increased performance. As well as the 8080’s seven registers and flags register, the Z80 had an alternate register set that duplicated them, two 16-bit index registers and additional instructions including bit manipulation and block copy/search.