Re: [TUHS] v6 RK05 bootloader question

On Dec 27, 2015, at 2:32 PM, Norman Wilson <norman(a)oclsc.org> wrote: Something of a tangent: In my early days with UNIX, one of the systems I helped look after was an 11/45. Normally we booted it from an SMD disk with a third-party RP-compatible contorller, for which we had a boot ROM. Occasionally, however, we wanted to boot it from RK05, usually to run diagnostics, occasionally for some emergency reason (like the root file system being utterly scrambled, or the time we ran that system, with UNIX, on a single RK05 pack, for several days so our secretaries could keep doing their troff work while the people who had broken our air-conditioning system got it fixed--all other systems in our small machine room had to stay shut down). There was no boot ROM for the RK05, but it didn't matter: one just did the following from the front-panel switches: 1. Halt/Enable to Halt 2. System reset (also sends a reset to the UNIBUS) 3. Load address 777404 4. Deposit 5. (watch lights blink for a second or so) 5. Load address 0 6. Halt/Enable to Enable 7. Continue 777404 is the RK11's command register. 5 is a read command. Resetting the system reset the RK11, which cleared all the registers; in particular the word count, bus address, and disk address registers. So when the 5 was deposited (including the bit 01, the GO bit), the RK11 would read from address 0 on the disk to address 0 in physical memory, then increment the word-count register, and keep doing so until the word count was zero after the increment. Or, in higher-level terms, read the first 65536 words of the disk into the first 65536 words of memory. Then starting at address 0 would start executing whatever code was at the beginning of memory (read from the beginning of the disk). Only the first 256 words (512 bytes) of the disk was really needed, of course, but it was harmless, faster, and easier to remember if one just left the word-count at its initial zero, so that is what we did. The boot ROM for the SMD disk had a certain charm as well. It was a quad-high UNIBUS card with a 16x16 array of diodes, representing 16 words of memory. I forget whether one inserted or removed a diode to make a bit one rather than zero. It's too bad people don't get to do this sort of low-level stuff these days; it gives one rather a good feel for what a bootstrap does when one issues the command(s) oneself, or physically programs the boot ROM. Norman Wilson Toronto ON