4 Jan
2017
4 Jan
'17
4:05 a.m.
On Tue, 3 Jan 2017, Jon Elson wrote:
So, does the LGP-30 have permanent storage on the
drum, or is it erased by a
magnet at every revolution?
Yes, the three timing tracks, and the upper half of the instruction
counter(!). The upper half containts the number of the following
sector and is used to locate the desired sector (either instruction or
data).
The G-15 modeled the drum just like a mercury delay
line, it had a write
head, a read head and a bar erase magnet on every line, so there was no gap
for the write head to turn on and off between words. The FF that drove the
write amp normally recirculated the data from the read head, except when a
word was being written. Each drum track had a long line (for program and
data) and a short line. Most of the short lines held 4 words, so they were
quickly accessible. One short line only held one word, that was the
accumulator.
The LGP-30 is similar, but the timing and long tracks (i.e. main memory)
are not recirculating. Hence the reason why a word is limited to 31 bits,
although the sector holds 32 bits. The last bit must be zero to reset the
read circuitry for the next sector.
Christian