Re: [TUHS] Does anybody recall how the TU10 bootstrap code actually operates?

Will Senn scripsit: As usual, they were standing on the shoulders of giants. The OS/8 bootstrap (on the PDP-8) for the RK8E disk controller was even more elegant, only two instructions long and requiring no further operator actions but "clear" and "start". The first instruction simply read the current block into the current memory address, which because of the "clear" read block 0 into memory location 0; the second was also a branch-to-self. But the real cleverness was that the bootstrap was placed at locations 30 and 31. As the disk block was read in by DMA, location 30 was overwritten with the "skip if disk is ready" instruction and location 31 with "branch to previous location". So first the CPU was idling in a one-instruction infinite loop, then in a two-instruction loop as long as the block was still loading, and finally would continue executing at location 32 when the disk bootstrap was fully loaded. The code there loaded the full device driver for the RK8E into reserved memory at 07600-07777 and jumped to the code within it that loaded the Keyboard Monitor (the shell) at location 0 and jumped into it. -- John Cowan http://www.ccil.org/~cowan cowan(a)ccil.org A rose by any other name may smell as sweet, but if you called it an onion you'd get cooks very confused. --RMS