The Refreshing process

The refresh scan for a 32/40 bit word on the Manchester Mark 1 machines was typically around 300/400 microseconds. Reading from and writing to a Main Store CRT was interleaved with refreshing scans, with each line of each CRT store being refreshed in turn cyclically. Operations not requiring Main Store access would be overlapped with refresh scans.

So for example on the Baby there was a simple "four beat rhythm" for a standard instruction:

  1. Refresh the next line (in the cycle) of Main Store; add 1 to the address of the current instruction
  2. Fetch the next instruction from Main Store
  3. Refresh the next line (in the cycle) of Main Store; decode the instruction
  4. Read/Write from/to Main Store as required; complete the instruction

Each beat took the same time (hence the official 1.2 millisecond instruction time). The complete 32 word Main Store would therefore be refreshed every 16 instructions, well before there was any significant discharge.

Note that where there was a 32-bit operation taking place in beat 4, i.e. A = A - S, the operation would still be completed within the beat, since the 32 bits were read from store serially and the combination with the accumulator value could take place serially at the same time.

On later Mark 1 machines the pattern of beats got increasingly more complex and irregular, but the same principle applied, i.e. of interleaving refresh scans with Main Store read/writes and overlapping refresh scans with other activity. Of course where there were a number of Main Store CRTs, each of the corresponding lines on each CRT would be refreshed simultaneously at each refresh scan.