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What is the most "C unfriendly" architecture for which a workable C compiler exists ?

Discussion in 'Embedded' started by Anton Erasmus, Jun 3, 2006.

  1. Hi,

    Since there is a thread on "C friendly" architectures, I thought it
    might be interested to find out what sort of horrible architectures
    compiler writers actually managed to create a C compiler for.
    (And what portion of the ISO C standard is/was not supported)
    I asked the question (emailed Plauger if I recall correctl) once, on
    why there is no "rotate" operator in C. ALL the architectures I have
    ever work on had a direct assembly instruction for rotating a
    register. The answer was that there were architectures which C
    supports on which a "rotate" instruction made no sense. An example
    was mention of an architecture that had NO integer support at the
    assembly level. Integer operations had to be emulated using floating
    point instructions.

    Anton Erasmus
    Anton Erasmus, Jun 3, 2006
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  2. Anton Erasmus

    Pete Fenelon Guest

    CCS and Hi-Tech have "C compilers" for the PIC12. From what I recall,
    neither support reentrancy, one or t'other doesn't support the right
    semantics for automatics (treats them as statics).

    While I'd probably never code in a C-like language on a PIC12 through
    choice, the option's there.

    Not sure if there've been C compilers for any 4-bit micros -- I think
    that's reaching the point where anything you write for such a small
    machine is likely to be better off in assembly language. It *could* be
    done, but I think humans will be better at generating code for such

    (Can't remember who observed in the other thread that the richer the
    register and instruction set of the processor, the less transparent
    he expected the generated code to be, because the compiler has more
    options as far as rearranging code, using different registers etc.
    are concerned - but he's absolutely right - particularly on chips
    with long pipelines where a lot of reordering goes on to
    keep them well-fed!)

    I think the ultimate challenge would be doing a compiler for the old
    Motorola 14500... if you could make it drive some RAM ;)

    Pete Fenelon, Jun 3, 2006
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  3. Anton Erasmus

    Tim Wescott Guest

    I'm not familiar with that processor, but trying to make an optimizing
    compiler for the ADSP 21xx would be a challenge -- if you ever need to
    answer the question "what's a non-orthogonal instruction set" just check
    that one out. Hand-optimizing assembly code would sometimes involve
    backtracking 20 instructions to change register choices, then trying it
    all again.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
    Tim Wescott, Jun 3, 2006
  4. Anton Erasmus

    Pete Fenelon Guest

    But that sounds the kind of code-gen you could try with all kinds of
    clever algorithms - anything up to and including the
    brute-force-and-ignorance of something like GNU superopt (which seems to
    have died the depth ;))

    The problem with the 14500 is that it's a 1-bit controller ;) --
    basically a replacement for relays ;)

    Pete Fenelon, Jun 4, 2006
  5. Anton Erasmus

    Walter Banks Guest

    There are lots of horrible architectures out there and HLL hide a
    multitude of their failings. The issues on implementing C on some
    architectures becomes less once the intended applications are
    well understood. The supporting C compilers often very well
    support intended applications. Take the PIC12 mentioned else
    where in this thread C removes the one thing that all assembly
    programmers do badly with and that is long term maintenance of
    ROM and RAM paging. The C compiler can track paging
    registers and RAM reuse very well. It is practical to implement
    a high level language on this type of processor.

    The arguments against using C on the PIC12 include lack of
    portability and lack of re-entrant code. (At least one compiler
    has implemented re-entrant code on a PIC 12 more as a check
    box issue rather than a practical use)

    We have found that code is quite portable to processors
    designed for a similar applications and less so for other processors.
    Very few very small embedded systems require re-entrancy
    in their applications and my experience is that it more of a C
    language requirement than an application implementation requirement.

    Rotates, adds and subtracts with carry are all instructions that
    support extended precision integer operations. It is interesting
    that the extensions in ISO C standards for embedded systems
    TR18037 makes it easy to create compilers that can support
    these instructions without resorting to assembly code.

    There have been worse processors than the Microchip PIC12
    proposed for use in embedded systems. The PIC 12 found its
    niche when it started being used in very small single purpose
    applications and then the HLL started to make sense for it.
    When we implemented a C compiler for the PIC12 tracking
    the paging and instruction side effects were the most time

    Walter Banks, Jun 5, 2006
  6. Anton Erasmus

    David Brown Guest

    The AVR tiny micros must be pretty close (the real, ramless tinies).
    The ISA itself is not too bad (except for the separate code and data
    spaces), but on the tiny's there is no RAM - only the 32 registers.
    There is a three-level hardware return stack, but no sensible way to
    make a data stack (although since the registers are memory-mapped to the
    data space, the single pointer register could theoretically be used as a
    data stack pointer). There's a C compiler from ImageCraft, and it's
    also possible to get gcc to work with it (although it involves a little
    bit of trickery).
    David Brown, Jun 5, 2006
  7. HP-1000 mini computers, (16 bit.) I am sure they are not the worst
    possible architecture for implementing C, but they had their quirks.
    (They were basically a 16 bit version of DEC's 12 bit PDP-8)

    A long time ago I ported the Small-C compiler to an HP-2113 and had to
    overcome the following:

    (a) Register set: Two accumulators, two index registers. (an
    architectural improvement over the original, that had only the two
    The X & Y index registers provided a few more operations and
    addressing modes at the cost of one or two more words, and one or two
    more memory access cycles. After trying a few alternate designs I
    found out that, in most cases, I could produce smaller and faster code
    if I did not use the index registers at all.

    (b) No hardware stack support - Must simulate stacks in software to
    support recursion or in-stack local variables, etc.

    (c) Limited addressing range inside the 32Kword address space. Opcodes
    could directly address operands in the same "page" were the opcode was
    located, or in page-0. I believe a page was a block of memory 1024
    words. It was not a relative address, which would have been a little
    more usable. Anything else could only be accessed using indirect
    addressing via a pointer in memory. Setting the MSB in a word address
    added (in hardware) an extra level of indirection. i.e, trying to
    fetch the word pointed by 0x8123 will fetch the word whose address was
    stored at address 0x0123. If that word also had the MSB set one more
    level of indirection will ensue, and so on.

    (d) Different addresses for 16-bit words and 8-bit bytes at the same
    memory location. There were a few byte and string oriented
    instructions using the index registers mentioned above, but they used
    a different addressing form. The basic architecture used word
    addressing. From 0x0000 to 0x7fff words. (32Kwords, 64Kbytes)
    The byte instructions used 0x0000 to 0xffff. The word at address N
    contained the bytes at (byte) addresses 2N and 2N+1.
    Again, after a few false starts trying to save memory, I just made
    char = int = 16 bit.

    I got the compiler to work, but after a while I abandoned it and
    returned to Ratfor, which offered the same or more functionality than
    the original Small-C. (sans recursion)
    Roberto Waltman, Jun 5, 2006
  8. Those are more or less obsolete by now...
    Hasn't been any new RAMless tiny's introduced in years
    Best Regards,
    Ulf Samuelsson

    This message is intended to be my own personal view and it
    may or may not be shared by my employer Atmel Nordic AB
    Ulf Samuelsson, Jun 5, 2006
  9. Anton Erasmus

    Alex Colvin Guest

    There was an early C compiler for the GE 635, which had (36-bit) word
    addressing. Characters (6-bit) and bytes (9-bit) could be addressed only
    via extra indirection words. Where an ordinary pointer fit in 18 bits and
    could be in a register, byte or character pointers needed 36 and could
    only be in memory.

    On the other hand, it was otherwise pretty conventional - flat address
    space (unless you were running Multics on a VM-enabled processor),
    call/return through registers or memory stacks, twos complement.
    Alex Colvin, Jun 7, 2006
  10. Has anyone actually used the 14500 ? - an example of a real application
    would be interesting.

    Jim Granville, Jun 7, 2006
  11. Can you clarify this - do you mean you will be (finally)
    able to (eg) RotateRight(arg), and RotateRightCY(arg) ?

    Jim Granville, Jun 7, 2006
  12. The real application is for a PLC-like controller. Think ladder logic.

    Best regards,
    Spehro Pefhany
    Spehro Pefhany, Jun 8, 2006

  13. The real application is for a PLC-like controller. Think ladder logic.[/QUOTE]

    I realise what Motorola intended it to be used for; I'm curious if
    anyone actually did a design using one ?

    I have seen PLCs using the Signetics 8X300, which relative to the
    14500, was an advanced chip :)

    Jim Granville, Jun 8, 2006
  14. Anton Erasmus

    Paul Burke Guest

    A customer had a 14500-based PLC board for a packing machine back in
    1981-ish when PLCs were expensive. Hex keypad program entry, EEPROM
    program memory. I used to write programs for it, using a photocopied
    coding sheet. It was an antique way of doing things even then. I've
    still got one of them around somewhere.

    Paul Burke
    Paul Burke, Jun 8, 2006
  15. Wow, thanks.
    Was that x8 EPROM, or wider ?
    Did you ever code counters in this ?

    I've been mulling over the idea of doing a 14500 in a CPLD,
    mainly as a teaching exercise.

    I can see simple ladder logic is do-able, but then wondered how to
    implement something like a counter, as the opcodes seem just a little
    short on that. No atomic complement data, for example,
    SKZ, but not SKNZ, and I'd be tempted to allow longer Skip sizes.....

    Maybe it needs a 14500++ :)

    Jim Granville, Jun 8, 2006
  16. Anton Erasmus

    Paul Burke Guest

    I've just dug it out of the glory hole... the one I've got here is a
    very late one, with 85 date codes on the chips. The program store is an
    XC2816 with an RS label (that was shortly after they'd stopped being
    Radiospares). The RAM is a 2147. Sadly I junked all the circuit diagrams
    etc. about 6 years ago.

    I never did counters, it was all straightforward relay replacement stuff.

    Paul Burke
    Paul Burke, Jun 8, 2006
  17. Anton Erasmus

    Walter Banks Guest


    TR 18037 allows direct access to the processors registers. If the
    condition codes are declared as a

    register volatile struct {
    int is_IRQ : 1;
    int disable_FIRQ : 1;
    int half_carry : 1;
    int disable_IRQ : 1;
    int N : 1;
    int Z : 1;
    int V : 1;
    int C : 1;
    } CC;


    c = a + CC.C + b;

    should now generate a

    load a
    adc b
    store c

    sequence. Similarily other instructions that
    involve condition code access can now be used.

    Walter Banks, Jun 8, 2006
  18. Anton Erasmus

    CBFalconer Guest

    Please don't use html and/or attachments in usenet.

    "Our enemies are innovative and resourceful, and so are we.
    They never stop thinking about new ways to harm our country
    and our people, and neither do we." -- G. W. Bush.
    "The people can always be brought to the bidding of the
    leaders. All you have to do is tell them they are being
    attacked and denounce the pacifists for lack of patriotism
    and exposing the country to danger. It works the same way
    in any country." --Hermann Goering.
    CBFalconer, Jun 8, 2006
  19. Thanks,
    Sounds like a worthwhile improvement (when supported:) , but it still
    does not allow access to RR/RRC opcodes ?

    Jim Granville, Jun 8, 2006
  20. You probably thought no one would ever ask about it :)
    I've worked out that counters are possible, with some small extensions.

    As time allows, I think I'll code a 14500, then re-map the 16 opcodes to
    remove dead spaces...
    Jim Granville, Jun 8, 2006
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