Inside the Intel 8085 Register File

Great, informative post by Ken Shirriff of the Intel 8085 register file design and implementation. He writes:

On the surface, a microprocessor’s registers seem like simple storage, but not in the 8085 microprocessor. Reverse-engineering the 8085 reveals many interesting tricks that make the registers fast and compact. The picture below shows that the registers and associated control circuitry occupy a large fraction of the chip, so efficiency is important. Each bit is implemented with a surprisingly compact circuit. The instruction set is designed to make register accesses efficient. An indirection trick allows quick register exchanges. Many register operations use the unexpected but efficient data path of going through the ALU.

While the 8085’s register complement is tiny compared to current processors, it has a solid register set by 1977 standards – about twice as many registers as the 6502. The 8085 has a 16-bit program counter, a 16-bit stack pointer, 16-bit BC, DE, and HL register pairs, and the 8-bit accumulator. The 8085 also has little-known hidden registers that are invisible to the programmer but used internally: the WZ register pair, and two 8-bit registers for the ALU: ACT and TMP.

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