2 Jul
2004
2 Jul
'04
11:16 a.m.
Tony> Or from ROM (originally diode matrix [1].
later on bipolar PROM
Tony> chips)
Tony> [1] Chosen, I am told, because the user could program it
Tony> without special equipment like a PROM programmer. All you
Tony> needed was a pair of cutters (the 'blank' boards shipped with
Tony> all the diodes fitted), and a soldering iron to correct errors.
That's certainly a benefit. Another likely reason is that PROMs
didn't exist yet (only, if anything, mask ROMs, which didn't make
Hmmm. The PDP11/45 used bipolar PROMs for the CPU and floating point
microcode. But the bootstrap board was a diode matrix. Now admittedly it
could have been designed for the 11/20 (which was a couple of years
earlier), and perhaps bipolar PROMs were not available then, but they
were certainly around in 1972.
sense in quantities less than 1 million). And even if
PROMs existed
back then, 256 diodes were likely to be cheaper.
The board I am thinking of has 32 locations of 16 bits, so 512 diodes. I
have reprogrammed a couple...
-tony