EDSAC as early as 1949 had an "initial orders" module. A boot-up ROM. There was a bank of rotary selector switches, to set the octal digits in each ROM word.
David Wheeler (of subroutine fame) figured out the most sensible thing to put in the tiny ROM was a paper tape loader and mini-assembler that loaded the rest of itself from paper tape. So at switch-on the machine would start reading the tape input and run. Programs could be written on the teletype with mnemonics and relative addresses in octal.
It seems a little strange that more advanced later machines like the original PDP-11 from 1970, couldn't "autostart" from ROM like that. You had to toggle in a bootloader.
I think that was because core memory was non-volatile. (EDSAC didn't use core; it had a refreshing DRAM-like delay line memory.) With core, you only had to toggle the bootloader into once and, provided your software didn't accidentally overwrite it, it was still there after a power cycle. So re-toggling in the boot loader was not really that common, despite how much lore surrounds it.
The EDSAC's "boot ROM" consisted of a bank of large electro-meachanical "uniselector" rotary switches, hardwired to contain the 31 instructions ("initial orders") of the tape loader.
Subsequent computers could have followed this "electro-mechanical ROM" approach, but I suppose it was rather bulky and inconvenient to change.
You can see what the uniselectors looked like here at 8:50
Of course programmable ROM chips would have to wait, so in the meantime the alternatives were either the toggle-it-in approach (still also used by the 1974 MITS Altair 8800), or other more compact types of handmade/hardcoded physical ROM such as magnetic core or diode matrices.
Typically daily. They were used for troubleshooting and repairing military avionics. They survived decades onboard ships, etc. They got temperamental in their later years, but we kept them going.
If you have any interest you can search up the (sparse) information on the AN/USM-484 Hybrid Test Set (or System).
retrac · · focus · HN ↗
David Wheeler (of subroutine fame) figured out the most sensible thing to put in the tiny ROM was a paper tape loader and mini-assembler that loaded the rest of itself from paper tape. So at switch-on the machine would start reading the tape input and run. Programs could be written on the teletype with mnemonics and relative addresses in octal.
It seems a little strange that more advanced later machines like the original PDP-11 from 1970, couldn't "autostart" from ROM like that. You had to toggle in a bootloader.
I think that was because core memory was non-volatile. (EDSAC didn't use core; it had a refreshing DRAM-like delay line memory.) With core, you only had to toggle the bootloader into once and, provided your software didn't accidentally overwrite it, it was still there after a power cycle. So re-toggling in the boot loader was not really that common, despite how much lore surrounds it.
HarHarVeryFunny · · focus · HN ↗
Subsequent computers could have followed this "electro-mechanical ROM" approach, but I suppose it was rather bulky and inconvenient to change.
You can see what the uniselectors looked like here at 8:50
<a href="https://www.youtube.com/watch?v=Yc945sNB0uA" rel="nofollow">https://www.youtube.com/watch?v=Yc945sNB0uA
Of course programmable ROM chips would have to wait, so in the meantime the alternatives were either the toggle-it-in approach (still also used by the 1974 MITS Altair 8800), or other more compact types of handmade/hardcoded physical ROM such as magnetic core or diode matrices.
CobaltFire · · focus · HN ↗
I was operating and maintaining several until about a decade ago.
HarHarVeryFunny · · focus · HN ↗
How often would you reboot them?
CobaltFire · · focus · HN ↗
If you have any interest you can search up the (sparse) information on the AN/USM-484 Hybrid Test Set (or System).