7 Oct
2007
7 Oct
'07
2:16 p.m.
On 7 Oct 2007 at 21:21, Tony Duell wrote:
Curiously, the HP9100 _calculator_ used 6 bit words.
This made sesne for
user programs (and keycodes), but numbers were stored with one BCD digit,
a sign bit and a blanking bit in each word (I think). The main processor
register was 6 bits wide too.
Let's not forget the venerable IBM 1620. 5 bits per word (Oh, okay,
there was a sixth parity bit, but it's not programmable). F8421. If
F was present over the low-order digit, it denoted a negative number.
A flag over the high-order digit signified where the number ended
(this was a variable word length machine). Certain non-BCD
combinations were reserved for other uses. 82, for example was a
record mark (which allowed you to deal with a group of words as a
unit); 842 was a group mark (used by the 1311 disk drive to transfer
groups of records; 84 was a numeric blank (display I/O had two modes;
numeric and alphanumeric). All of the above could also be flagged
and treated as part of a varaible-length word. In addition, a flag
over the low-order digit of an address signified indirection (if the
system was so equipped). Words and addresses were addressed by the
low-order (highest address) digit; instructions, records and groups
were addressed by the high order (lowest address) position.
Cheers,
Chuck