28 Feb
2008
28 Feb
'08
1:12 p.m.
On 28 Feb 2008 at 10:19, Geoffrey Bunza wrote:
On the note of the CDC 6600 machines, et al., There
was a peripheral
processor which was used to boot the mainframe, and it had many rows of
toggle switches, in groups of 18 or 36 if I remember right, which
either contained the boot code or controlled the boot. The peripheral
processor was a computer in its own right by any standards.
Well, actually a tenth of a computer. The basic cycle time of the
mainframe was 100 nsec. There were 10 PPUs in a standard
configuration, with a cycle time of 1 usec. In other words, each PPU
was little more than 4K of 12 bit core, a P-counter and an 18-bit
accumulator (that's over-simplifying things a bit, but it'll give you
the idea). The "guts" (instruction decode and execution) was time-
sliced around the 10 PPUs. This was sometimes referred to as the
PPU's slot "in the barrel". All I/O channels were accessible from
any PPU and any PPU could access central memory. So, if a driver or
bit of hardware malfunctioned, the PPU might hang, but the system
could keep on running. Similarly, if a PPU was hung on an empty
channel, it was often possible to break it loose from the operator's
console.
The deadstart panel itself was a 12x12 matrix of switches (12 words
of 12 bits). There were some other switches controlling diagnostic
mode, and control of various options (e.g. CEJ/MEJ); many systems had
a key-operated switch which could disable the deadstart panel to keep
rogue programmers from sneaking time on the system. :)
Deadstart was quite simple. Each PPU had its P-counter forced to
0000 and a block input instruction from the channel corresponding to
the position of the PPU in the barrel. (i.e., 0-9). The contents of
the deadstart panel switches were output to channel 0 after an
initial word of zero and the channel disconnected. PPU 0 then began
executing at location 1 and could then proceed to read the remainder
of the OS bootstrap and transmit resident programs to each of the
other PPUs. No blinkenlights and no ROM involved at all--either the
system started or you had blank screens staring back at you. Time to
get out the scope...
10 is a magic number for the 6600. Memory is interleaved 10-way,
there are 10 PPUs, 10 characters per CPU word, 10 functional units...
Later implementations of the architecture dropped the idea of the
barrel and provided a complete processor for each PPU.
Cheers,
Chuck