Re: [TUHS] A New History of Modern Computing - my thoughts

Hi Bakul, The ARM chip came about because the UK's Acorn Computers couldn't find a decent cost:performance 16-bit CPU to replace the 6502 in new models. Furber and Wilson realised from a visit to Bill Mensch's WDC that chip design could be done by a small outfit. The recent UCB reports on RISC suggested a simple design could still have high performance so they set about skipping 16-bit CPUs and rolling their own 32-bit RISC CPU, the Acorn RISC Machine. The point of that bit of history is they were not chip designers, but knew electronics and programming. Wilson designed the ARM's instruction set and it was a delight to code: very orthogonal, and every instruction had four-bits of condition-flag test, e.g. Carry Set, and a bit to indicate if this instruction should set the condition flags. Thus several instructions in a row could test the condition flags set by an instruction a few earlier and unaltered since; this cut the need for quite a few branches. I think Wilson did such a good job because she had coded extensively in assembler on several different architectures. Not just the odd device driver or context-switch but 16 KiB of 6502 instructions which were BBC BASIC, a structured BASIC with WHILE, PROC, integers, floats, etc. (The BASIC ROM remains a good test of any 6502 emulator today because of all the corner cases the hand-written assembly exercised.) This was just for the BASIC interpreter; the OS, file system, etc., were all in other 16 KiB ROMs of hand-written assembly. The ability to add a co-processor to the BBC Microcomputer meant Z80 and other implementations followed. So with all that experience it's not surprising that the instruction set, though RISC, gave just what the assembly writer wanted, nothing more, whilst being easy to learn and remember. I don't think a hardware-chip designer could have done such a good job. The later pressure to drop 32-bit instructions for a mixture of 16-bit and 32-bit due to mobile's small flash capacity beget Thumb, still used today as Thumb-2, which dropped much of the nice features which made hand-written assembly such a pleasure, but then by then compilers were better quality and more common. I agree with the sentiment, but it sounds quite a big leap for most programmers. I knew assembly and logic but not how to fill the gap to create a CPU. The popular book from a few years ago which plugged the hole for me was ‘The Elements of Computing Systems’, which I see has now had a second edition: ISBN 0-262-53980-2, https://amzn.to/3xE0IZo It's a book of two halves: the first builds a simple CPU for a primitive computer from nothing but NAND gates; the second half writes an assembler, then a compiler for a language targeting a VM and then implements the VM on the CPU built in part one. It's the reader who has to do all this. Initially in the book's HDL run against the provided simulators and test cases. Then in the programming language of his choice as just text I/O is required. So to start the other binary-operator logic gates have to be build with NANDs, then multiplexers, etc. There's a target of how many gates to use so an efficient design has to be produced. By chapter two we're implementing the ALU. After the sequential logic of chapter three the instruction set is introduced in chapter four. It proceeds at pace, just giving the barest tuition needed to understand each conceptual part. Any discussion of subtleties or alternative approaches is eschewed for weightier textbooks. I think it's a better introduction to CPU design and from that one can read up on Verilog, etc., and start experimenting with little CPU designs on FPGAs. -- Cheers, Ralph.