On 5/16/20, Steffen Nurpmeso <steffen@sdaoden.eu> wrote:
>
> Why was there no byte or "mem" type?
These days machine architecture has settled on the 8-bit byte as the
unit for addressing, but it wasn't always the case. The PDP-10
addressed memory in 36-bit units. The character manipulating
instructions could deal with a variety of different byte lengths: you
could store six 6-bit BCD characters per machine word,
Was this perhaps a typo for 9 4-bit BCD digits? I have heard that a reason for the 36-bit word size of computers of that era was that the main competition at the time was against mechanical calculator, which had 9-digit precision. 9*4=36, so 9 BCD digits could fit into a single word, for parity with the competition.
6x6-bit data would certainly hold BAUDOT data, and I thought the Univac/CDC machines supported a 6-bit character set? Does this live on in the Unisys 1100-series machines? I see some reference to FIELDATA online.
I feel like this might be drifting into COFF territory now; Cc'ing there.
or five ASCII
7-bit characters (with a bit left over), or four 8-bit characters
(ASCII plus parity, with four bits left over), or four 9-bit
characters.
Regarding a "mem" type, take a look at BLISS. The only data type that
language has is the machine word.
> +getfield(buf)
> +char buf[];
> +{
> + int j;
> + char c;
> +
> + j = 0;
> + while((c = buf[j] = getc(iobuf)) >= 0)
> + if(c==':' || c=='\n') {
> + buf[j] =0;
> + return(1);
> + } else
> + j++;
> + return(0);
> +}
>
> so here the EOF was different and char was signed 7-bit it seems.
That makes perfect sense if you're dealing with ASCII, which is a
7-bit character set.
To bring it back slightly to Unix, when Mary Ann and I were playing around with First Edition on the emulated PDP-7 at LCM+L during the Unix50 event last USENIX, I have a vague recollection that the B routine for reading a character from stdin was either `getchar` or `getc`. I had some impression that this did some magic necessary to extract a character from half of an 18-bit word (maybe it just zeroed the upper half of a word or something). If I had to guess, I imagine that the coincidence between "character" and "byte" in C is a quirk of this history, as opposed to any special hidden meaning regarding textual vs binary data, particularly since Unix makes no real distinction between the two: files are just unstructured bags of bytes, they're called 'char' because that was just the way things had always been.
- Dan C.