11 Jun
2026
11 Jun
'26
1:56 p.m.
On Jun 11, 2026, at 4:34 PM, Fred Cisin via cctalk
<cctalk(a)classiccmp.org> wrote:
On Thu, 11 Jun 2026, Tom Gardner via cctalk wrote:
https://www.bitsavers.org/pdf/ibm/305_ramac/22-6264-1_305_RAMAC_Manual_of_O…
has the answer:
50 surfaces, 100 tracks per surface, 10 sectors per track, 100 characters per sector.
Multiples of 10 show up in later products too, like the 1311 disk packs for the IBM 1620.
Then of course there's weird stuff like the IBM/360 disks with their variable length
sectors. Or the CDC mainframes with 322 12-bit words per sector. Or Electrologica where
you could choose one of five sector sizes on a per-track basis. Or the DEC RS64 with 64
byte sectors, or the word-addressable RF11.
paul
Ramac had fifty 2 foot diameter double sided
platters, and could hold a
total of about 5MB.'
The IBM 350 disk storage capacity was 5 million 6-bit characters == 3.75
MB.
Thank you, I couldn't remember how many bytes per track to calculate accurately. They
might not have been multiples of 128? Long, long ago, I was told 100 bytes per sector?