17 Feb
2013
17 Feb
'13
4:02 p.m.
Michael Thompson wrote:
It is an interesting machine because it is microcoded
with read-only
core memory. I am not sure why they used core for the microcode. Maybe
just a board with a bunch of diodes would have done the same thing and
would have been a lot simpler?
Implementing the PDP-9 control store with diodes would have required a
fairly large board, as it would have needed locations for 2304 diodes.
Assuming that the microcode had an even distribution of ones and zeros,
it would have needed 1152 diodes. I'm reasonably sure that using core
was more compact and less expensive.
In the PDP-9, each core of the core rope memory stores 64 bits, unlike
conventional read/write core memory, where each core stores only a
single bit. Core rope memory uses the core as a transformer, and does
not depend on the hysteresis loop of the core nor attempt to change the
direction of magnetization of the core. There are 64 words of control
store, so there are 64 wires. Each of those wires goes through some of
the 36 cores, and around others. When a pulse is sent down a word line,
in only the cores that it passes through, it induces a pulse in a sense
line.
The same general technique was used for the lowest-level control store
of the HP 9100 calculator, and the program store of the Apollo Guidance
Computer. IBM used similar technology for the "TROS" control store of
some System/360 processors and peripheral controllers.
Eric