4 ms·

The funny thing about the PDP-11 instruction set is how ridiculously simple it is to write a C compiler for it… the C operands map almost 1 to 1 to PDP-11 instr

The funny thing about the PDP-11 instruction set is how ridiculously simple it is to write a C compiler for it… the C operands map almost 1 to 1 to PDP-11 instructions. At least that is what my compilers writing class taught me 25 years ago ;-)

kencausey 6y ago
Probably you are well aware of this, but arguably the causation runs the other way: https://en.wikipedia.org/wiki/C_(programming_language)#History https://en.wikipedia.org/wiki/C_(programming_language)#Histo...
- hazeii 6y ago
  As a PDP-11 assembly programmer who migrated to C, this one has caught me out too because it does 'feel' a very natural explanation. Possibly it's because both products came from the ideas that were in circulation at the time - or maybe it's that PDP-11 assembly programmers who moved to C found it easy (given pointers with pre/post-increment/decrement addressing is baked into the instruction set).
  - kbob 6y ago
    To this day, I have a subconscious desire to use postfix increment and prefix decrement because of the PDP-11's addressing modes. "p++, --p"
simias 6y ago
I'm not really seeing it, at least not in this opcode list. As far as I can tell the C operands are fairly simple and basic (add, subtract, complement, and, or, xor etc...). They would almost always map to a single instruction in MIPS, ARM32, ARM64, Z80 etc... So I'm not convinced that there's a direct correlation here, one way or the other. Conversely if C was really deeply influenced by the PDP-11 ISA then why isn't there a single "bit set", "bit clear" or "rotate right" operator in C? Those require multiple operators in C but a single opcode here. Rotation in particular is a bit of a pain to code in C since you need to know the size of the integer you're rotating. I always thought that it was an oversight not to have built-in support for rotations in C. The only thing that really stands out as being fairly C-ish is the auto-incrementing and -decrementing addressing modes which map well to p++ and friends (although ++p needs two opcodes as far as I can tell) but it's a feature that's available on other ISAs.
- zwegner 6y ago
  Just curious: is there a use case for rotations where the integer size isn't important? It's perhaps a bit annoying to need size-specific rotation functions, but all the uses of rotations I know of need specific integer sizes anyways.
  - simias 6y ago
    You're right of course, I just mean that it makes it a bit verbose and error-prone to write in full "(i >> (32 - shift)) | (i << shift))". I need to triple check that I'm not off-by-one and that all the operators are in the right direction every time I write this. If you had rotation operators like, say, "<|" you could write "i <| shift" instead and that'd be a lot nicer to read and write IMO.
- inkyoto 6y ago
  You would need to look into the addressing modes at length. * --i = * j++ is directly a «mov @(r0)+, -(r1)» (provided * j is in r0 and * i is in r1) whereas * i++ = * j-- is a «mov @-(r0),@(r1)+»; i[4] = 1 is «mov #1, @4(r1)» and * (int * )040 |= 01 is «bis 01, @#040». A function call via a function pointer (that is, say, stored in r2) is a «jsr pc, @(r2)» etc etc. Most of the C pointer operations map onto single PDP-11 instructions and can be used even with the program counter register (which is r7).

kencausey 6y ago

Probably you are well aware of this, but arguably the causation runs the other way: https://en.wikipedia.org/wiki/C_(programming_language)#History https://en.wikipedia.org/wiki/C_(programming_language)#Histo...

hazeii 6y ago
As a PDP-11 assembly programmer who migrated to C, this one has caught me out too because it does 'feel' a very natural explanation. Possibly it's because both products came from the ideas that were in circulation at the time - or maybe it's that PDP-11 assembly programmers who moved to C found it easy (given pointers with pre/post-increment/decrement addressing is baked into the instruction set).
- kbob 6y ago
  To this day, I have a subconscious desire to use postfix increment and prefix decrement because of the PDP-11's addressing modes. "p++, --p"

simias 6y ago

I'm not really seeing it, at least not in this opcode list. As far as I can tell the C operands are fairly simple and basic (add, subtract, complement, and, or, xor etc...). They would almost always map to a single instruction in MIPS, ARM32, ARM64, Z80 etc... So I'm not convinced that there's a direct correlation here, one way or the other. Conversely if C was really deeply influenced by the PDP-11 ISA then why isn't there a single "bit set", "bit clear" or "rotate right" operator in C? Those require multiple operators in C but a single opcode here. Rotation in particular is a bit of a pain to code in C since you need to know the size of the integer you're rotating. I always thought that it was an oversight not to have built-in support for rotations in C. The only thing that really stands out as being fairly C-ish is the auto-incrementing and -decrementing addressing modes which map well to p++ and friends (although ++p needs two opcodes as far as I can tell) but it's a feature that's available on other ISAs.

zwegner 6y ago
Just curious: is there a use case for rotations where the integer size isn't important? It's perhaps a bit annoying to need size-specific rotation functions, but all the uses of rotations I know of need specific integer sizes anyways.
- simias 6y ago
  You're right of course, I just mean that it makes it a bit verbose and error-prone to write in full "(i >> (32 - shift)) | (i << shift))". I need to triple check that I'm not off-by-one and that all the operators are in the right direction every time I write this. If you had rotation operators like, say, "<|" you could write "i <| shift" instead and that'd be a lot nicer to read and write IMO.
inkyoto 6y ago
You would need to look into the addressing modes at length. * --i = * j++ is directly a «mov @(r0)+, -(r1)» (provided * j is in r0 and * i is in r1) whereas * i++ = * j-- is a «mov @-(r0),@(r1)+»; i[4] = 1 is «mov #1, @4(r1)» and * (int * )040 |= 01 is «bis 01, @#040». A function call via a function pointer (that is, say, stored in r2) is a «jsr pc, @(r2)» etc etc. Most of the C pointer operations map onto single PDP-11 instructions and can be used even with the program counter register (which is r7).