4 ms·

6502 is a better "toy" asm than the Z80, but that's not saying much. It's not even obviously better than the AVR 8-bit insn set. As far as more modern platfor

6502 is a better "toy" asm than the Z80, but that's not saying much. It's not even obviously better than the AVR 8-bit insn set. As far as more modern platforms, I think there is a strong case for teaching RISC-V over something like MC68k. RISC-V can be very simple and elegant (in the base integer insn set) while still being very similar to other modern architectures like ARM, Aarch-64 and MIPS. It's also available in both 32 and 64-bit varieties, and the official documentation is very accessible. MC68k just has tons of quirks which aren't really going to be relevant in the modern day and age. (About the only thing it has going for it is the huge variety of hardware platforms that happened to make use of it, many of which still have thriving retro communities around them. But that's more of a curiosity as opposed to something genuinely relevant.)

adrian_b 2y ago
Some people believe that RISC-V is simple and elegant, other people believe that RISC-V is one of the worst and ugliest instruction-set architectures that have ever been conceived. Usually the first class of people consists of people with little experience in assembly programming and the second consists of those who had experience in using multiple ISAs for assembly programming. Using RISC-V for executing programs written in high-level programming languages is fine. It should be noted that in the research papers that have coined the name RISC, where there was a list of properties defining what RISC means, one of them was that RISC ISAs were intended to be programmed exclusively in high-level languages and never in assembly language. From this point of view, RISC-V has certainly followed the original definition of the term RISC, by making the programming in assembly language difficult and error prone. On the other hand, learning assembly programming with RISC-V is a great handicap because it does not expose the programmer to some of the most fundamental features of programming in an assembly language, because RISC-V does not have even many features that existed in Zilog Z80 or in vacuum-tube computers from 70 years ago, like integer overflow detection and indexed addressing. Someone who has learned only RISC-V has an extremely incomplete image about how a normal instruction set architecture looks like. When faced with almost anyone of the several thousands of different ISAs that have been used during the last three quarters of a century a RISC-V assembly programmer will be puzzled. 6502 is also too quirky and unusual. I agree that among the old ISAs DEC PDP-11 or Motorola MC68000 are better ISAs for someone learning to program in an assembly language from scratch. Writing assembly programs for any of these 2 is much simpler than writing good assembly programs for RISC-V, where things as simple as doing a correct addition are quite difficult (by correct operation I mean an operation where any errors are handled appropriately).
- bjourne 2y ago
  Great flamebait ;) what makes riscv suitable for teaching is that the ISA is trivial to map to hardware. This is because it is so regular. Students with zero experience can design simple riscv cpus in their first asm course. More experienced students can design cpus with pipelining, branch prediction, ooo execution, etc. Old ISAs from the 1980s are way more difficult.
  - adrian_b 2y ago
    RISC-V has been designed with the exact goal for being an ISA easy to implement in hardware, so that students will be able to do this. For this purpose, RISC-V is excellent. The RISC-V ISA has not been designed as an efficient method for encoding computer programs and even less for being easy to program in assembly language. When programming in assembly language, a complex ISA is bad, because one cannot hold in mind all its peculiarities, but a too simple ISA is even worse, because every simple operation must be implemented by a complex sequence of instructions that is easy to forget and to get wrong. I wonder if any of those who claim that the RISC-V ISA is simple can remember without searching the documentation how to implement the checks for integer overflow after each arithmetic operation. The didactic examples of using RISC-V usually do not check for any errors, which is not acceptable in any critical application. I find funny that sometimes the same people who claim that the unsafe RISC-V ISA is good also claim that C and C++ are bad, because with default compilation options they are unsafe in comparison with e.g. Rust.
    - pcwalton 2y ago
      > I wonder if any of those who claim that the RISC-V ISA is simple can remember without searching the documentation how to implement the checks for integer overflow after each arithmetic operation. I mean, I can't give you the Hacker's Delight magic numbers for expressing integer division by a constant via multiplication on AArch64 or x86-64 off the top of my head either, but that's what we have compilers for. The fact that you sometimes have to look up how to do simple things is just part of programming.
      - deleted 2y ago

adrian_b 2y ago

Some people believe that RISC-V is simple and elegant, other people believe that RISC-V is one of the worst and ugliest instruction-set architectures that have ever been conceived. Usually the first class of people consists of people with little experience in assembly programming and the second consists of those who had experience in using multiple ISAs for assembly programming. Using RISC-V for executing programs written in high-level programming languages is fine. It should be noted that in the research papers that have coined the name RISC, where there was a list of properties defining what RISC means, one of them was that RISC ISAs were intended to be programmed exclusively in high-level languages and never in assembly language. From this point of view, RISC-V has certainly followed the original definition of the term RISC, by making the programming in assembly language difficult and error prone. On the other hand, learning assembly programming with RISC-V is a great handicap because it does not expose the programmer to some of the most fundamental features of programming in an assembly language, because RISC-V does not have even many features that existed in Zilog Z80 or in vacuum-tube computers from 70 years ago, like integer overflow detection and indexed addressing. Someone who has learned only RISC-V has an extremely incomplete image about how a normal instruction set architecture looks like. When faced with almost anyone of the several thousands of different ISAs that have been used during the last three quarters of a century a RISC-V assembly programmer will be puzzled. 6502 is also too quirky and unusual. I agree that among the old ISAs DEC PDP-11 or Motorola MC68000 are better ISAs for someone learning to program in an assembly language from scratch. Writing assembly programs for any of these 2 is much simpler than writing good assembly programs for RISC-V, where things as simple as doing a correct addition are quite difficult (by correct operation I mean an operation where any errors are handled appropriately).

bjourne 2y ago
Great flamebait ;) what makes riscv suitable for teaching is that the ISA is trivial to map to hardware. This is because it is so regular. Students with zero experience can design simple riscv cpus in their first asm course. More experienced students can design cpus with pipelining, branch prediction, ooo execution, etc. Old ISAs from the 1980s are way more difficult.
- adrian_b 2y ago
  RISC-V has been designed with the exact goal for being an ISA easy to implement in hardware, so that students will be able to do this. For this purpose, RISC-V is excellent. The RISC-V ISA has not been designed as an efficient method for encoding computer programs and even less for being easy to program in assembly language. When programming in assembly language, a complex ISA is bad, because one cannot hold in mind all its peculiarities, but a too simple ISA is even worse, because every simple operation must be implemented by a complex sequence of instructions that is easy to forget and to get wrong. I wonder if any of those who claim that the RISC-V ISA is simple can remember without searching the documentation how to implement the checks for integer overflow after each arithmetic operation. The didactic examples of using RISC-V usually do not check for any errors, which is not acceptable in any critical application. I find funny that sometimes the same people who claim that the unsafe RISC-V ISA is good also claim that C and C++ are bad, because with default compilation options they are unsafe in comparison with e.g. Rust.
  - pcwalton 2y ago
    > I wonder if any of those who claim that the RISC-V ISA is simple can remember without searching the documentation how to implement the checks for integer overflow after each arithmetic operation. I mean, I can't give you the Hacker's Delight magic numbers for expressing integer division by a constant via multiplication on AArch64 or x86-64 off the top of my head either, but that's what we have compilers for. The fact that you sometimes have to look up how to do simple things is just part of programming.
    - deleted 2y ago