Digging deep into how the 8085 processor’s registers were designed

8085-register-reverse-engineering

Hardware design enthusiasts should already be salivating just looking at this image. But [Ken Shirriff's] write-up on how the 8085 processor’s registers were designed will put you in silicon reverse-engineering heaven. He manages to get to the bottom of the tricks the designers used to make register access as efficient as possible, like routing some through the ALU on their path elsewhere.

We’re certainly not experts in studying dies like the one seen above. Luckily [Ken] does a great job of zooming in on important parts, then dissecting how they work by representing the silicone image as a functional flow chart. One of the parts which we found most interesting is the WZ temporary registers. These are a set of internal registers that are not accessible to the programmer. They’re only used internally by the chip. They act as temporary storage for multiple operand functions, and also hold register addresses for a handful of instructions (JMP, CALL, RST, etc.).

If you’re more interested in how images of these chips are attained you should do some searching on Hackaday. Just last week we featured one such project in a links post.

[via Reddit]

How the 8085 ALU is structured

8085-alu-reverse-engineering

This is a microscopic photograph of an 8085 processor die. [Ken Shirriff] uses the image in his explanation of how the ALU works. It is only capable of five basic operations: ADD, OR, XOR, AND, and SHIFT-RIGHT. [Ken] mentions that the lack of SHIFT-LEFT is made up for by adding the number to itself which has the effect of multiplying a number by two; the same mathematical function performed by a shift operation.

His post details the gate arrangement for each ALU operation. This is clear and easy to follow, and was based on reverse engineering work already done by a team who meticulously decapped and photographed the dies.

Not long ago this explanation would have been voodoo to us. But we worked our way through The Elements of Computing Systems text-book by following the online Nand to Tetris course. It really demystifies the inner working of a chip like the 8085.

Now if you really want to understand this ALU you’ll build it for yourself inside of Minecraft.

[Thanks Ed]