18 Sep
2015
18 Sep
'15
10:15 p.m.
Simple answer. The control store functionality had to fit in the available space
on one hex
sized card, and 16K density DRAM was the only option. Allocating additional
space to use
4K or 1K SRAM would have pushed the CPU design to an additional board, which was not
a design option at the time.
The 11/730 was released in 1982 (according to Wikipedia which I don't take a gospel,
but
I have no other source of information for this). The PERQ 1a was certainly out by then, it
had
a control store using 16K*1 SRAM chips. OK 20 pin as against 16 pin packages, but I
don't
think the 11/730 boards are that tighly packed (anyway, using SRAM would probably have
eliminated some support devices like address multiplexers).
-tony