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Why the Z-80's Data Pins Are Scrambled
An anonymous reader writes "The Z-80 microprocessor has been around since 1976, and it was used in many computers at the beginning of the PC revolution. (For example, the TRS-80, Commodore 128, and ZX Spectrum.) Ken Shirriff has been working on reverse engineering the Z-80, and one of the things he noticed is that the data pins coming out of the chip are in seemingly random order: 4, 3, 5, 6, 2, 7, 0, 1. (And a +5V pin is stuck in the middle.) After careful study, he's come up with an explanation for this seemingly odd design. "The motivation behind splitting the data bus is to allow the chip to perform activities in parallel. For instance an instruction can be read from the data pins into the instruction logic at the same time that data is being copied between the ALU and registers.
[B]ecause the Z-80 splits the data bus into multiple segments, only four data lines run to the lower right corner of the chip. And because the Z-80 was very tight for space, running additional lines would be undesirable. Next, the BIT instructions use instruction bits 3, 4, and 5 to select a particular bit. This was motivated by the instruction structure the Z-80 inherited from the 8080. Finally, the Z-80's ALU requires direct access to instruction bits 3, 4, and 5 to select the particular data bit. Putting these factors together, data pins 3, 4, and 5 are constrained to be in the lower right corner of the chip next to the ALU. This forces the data pins to be out of sequence, and that's why the Z-80 has out-of-order data pins."
[B]ecause the Z-80 splits the data bus into multiple segments, only four data lines run to the lower right corner of the chip. And because the Z-80 was very tight for space, running additional lines would be undesirable. Next, the BIT instructions use instruction bits 3, 4, and 5 to select a particular bit. This was motivated by the instruction structure the Z-80 inherited from the 8080. Finally, the Z-80's ALU requires direct access to instruction bits 3, 4, and 5 to select the particular data bit. Putting these factors together, data pins 3, 4, and 5 are constrained to be in the lower right corner of the chip next to the ALU. This forces the data pins to be out of sequence, and that's why the Z-80 has out-of-order data pins."
Why didn't they just ask Federico Faggin? According to Wikipedia, he's still alive.
The Z-80 was a great chip and overall system design supported by very capable support peripheral chips in that family. The best part of working with it, is Zilog's Documentation, which was very well written and demonstrated the consistency of the entire product line (in terms of it's functional programming interfaces). There really is not any need to 'reverse engineer' the chip, everything you need to know is freely available already. I think this article and author means to say "Here are some plausible possible reasons behind some of these physical implementation decisions...".
I think all first year computer science / programming / engineering students should be introduced to this and learn how to write programs for this environment first before moving on to modern systems. True power is being able to write useful stuff with only 64kb of ram and 1mhz of processor, and have it run in an acceptable time frame, and taking those skills and scaling up today's multi-core/ multi-gigahertz/multi-gigabyte address spaces.
Not sure how sarcastic you're trying to be. I know nothing about chip design. I read the article and learned something. These are the type of articles that reflect what I believe slashdot is about. So it's something that I came here to learn about. There's no NSA/Snowden/loss of freedom/political drama/clickbait/ in the article, so there's no trolls. Win win.
Now if only there were more comments to add to the understanding.
Politics; n. : A religion whereby man is god.
Articles like this, makes me warm and fuzzy all over, probably because I'm an old geezer in comparison to kids of today, but I think it's very important for anyone serious about hardware development and/or software development to dive into the past once in a while, it's a great way to learn simplicity and how the hardware inside our relatively complicated devices of today really works.
/. back to the roots.
I'm a moderator of a major international electronics forum, and I don't have the number on just how many times the young generation feel completely lost when they're fresh out of school, trying to understand very complex structures. They either lack understanding of general electronics, or how the microprocessor works with different layers, ram, rom (especially embedded systems when they are working with complex IDE's with a maze of classes & libraries), they simply forget how the hardware works, and get to focus too much on programming.
I understand exactly that frustration, especially since this old geezer was lucky enough to grew up with basic home computers like the Commodore 64, Zx81 (Z80 cpu), Spectrum, Oric, Dragon 32, BBC etc. We often did our own hardware modifications, made fast I/O port load&save systems ourselves because we had a basic understanding of how the innards worked, and it really wasn't rocket science.
Sometimes it is relevant to take a step back in time (Like this article does, explaining some of the oddities with the Z80 processor), and spark interest in these old CPU's and their systems & possible uses even today. As an example, I have a HUGE stash of Micro-Controllers in my workshop, these are an absolute GEM to me. Why? Because they are very simple to work with. Like the good old Commodore 64 or ZX 81 - they don't have advanced hardware layers where you have to do special addressing to access certain memory areas or have to be kind to the operating system in order to write something to control your hardware (homemade or otherwise), it's as simple as writing a few pokes into memory...and you can turn on/off some external units such as relays, lights - or read on/off states from your sensors...maybe build your own satellite tracker the easy way, or control your homemade lawnmover unit.
And we still have VAST amounts of these MCU's unused all over the world, these are SUPER USEFUL (if you didn't get the above, think standalone apps...like each MCU was an app for a specific task). Many of these CPU's (MCU usually comes with internal memory/Ram/Rom/Flash/ and the most important part...an I/O) ready to use, just program it...and watch it go. If the kids of today understood this, they'd have a BLAST programming these (just watch the maker society with their modern versions...Arduino etc.) and the sky's the limit.
More articles like these thanks, brings
What this world is coming to - is for you and me to decide.
It's totally non-news.
It's basically a trivial fact that could probably have been answered just by ASKING anyone who actually knew about the details of the Z-80 development, but some dude decided to puzzle it out himself, slow /. news day, and bam, here's the article.
Hold on... I just had an epiphany about why manhole covers are round - give me a sec, I think I'm going to submit it...