B (cross)compiler for the PDP-8

[Obsolescence]

Hi,

I very much enjoyed this just-released B compiler designed for the PDP-8, written by Robert Clausecker. Giving me another well-spent sleepless night ;)

https://github.com/fuzxxl/8bc

Why is it enjoyable? For me --

(1) this is a good short overview of the PDP-8 instruction set and what it means for a compiler,

(2) it shows a super-compact way of making a compiler with lex and yacc,

(3) B is like C, so simple to learn.

(4) As this was made as an educational tool, the source code is written for legibility. Even the resulting assembly code coming out of the compiler is easy to read.

(5) Another proof that universities still teach on PDP-8s!

Attached is a PDF that's worth reading even if you don't plan to code in B. Read a little, learn a lot ;)

Kind regards,

Oscar.

Example: sort in B:

/* sort input lines alphabetically */
main() {
    extrn readall, mklines, sort, puts, end;
    auto i 0, nlines, input, lines;

    input = &end;
    lines = readall(input);
    nlines = mklines(lines, input);
    sort(lines, nlines);
    while (i < nlines)
        puts(lines[i++]);
}

/* read the input into buffer and return a pointer to its end */
readall(buffer)
{
    extrn getchar;

    while ((*buffer++ = getchar()) != '*e')
        ;

    return (buffer);
}

/*
 * Split buffer into *e terminated lines.  Store pointers to the
 * lines into lines and return the number of lines.
 */
mklines(lines, buffer)
{
    auto i 0, nlines 0;

    while (1) {
        lines[nlines++] = buffer + i;

        while (buffer[i] != '*n' & buffer[i] != '*e')
            i++;

        if (buffer[i] == '*e')
            return (nlines);
        else
            buffer[i++] = '*e';
    }
}

/* compare strings alphanumerically.  Return -1, 0, or 1 */
strcmp(a, b)
{
    auto i 0;

    while (a[i] == b[i])
        if (a[i++] == '*e')
            return 0;

    return (a[i] > b[i]) - (a[i] < b[i]);
}

/* sort string pointers in lines by string order */
sort(lines, nlines)
{
    extrn strcmp;
    auto i, j 0, tmp;

    /* insertion sort */
    while (j < nlines) {
        i = j++;
        tmp = lines[i];

        while (i > 0 & strcmp(lines[i-1], tmp) == 1) {
            lines[i] = lines[i - 1];
            --i;
        }

        lines[i] = tmp;
    }
}

/* print string followed by a newline */
puts(str)
{
    extrn putchar;

    while (*str != '*e')
        putchar(*str++);

    putchar('*n');
}

[Obsolescence]

On the B compiler, a video presentation is now online: (link)

A very accessible introduction to the PDP-8 instruction set and the challenges of making a compiler to it.