Disk capacity????

[nwbe...@umich.edu]

I have a hard disk made by Caelus
---ENTIRE LABEL---
MODEL CMI-HD-12 SAI 703
P/N 302204-059
100 TPI, 2200 BPI
12 SECTOR
---END LABEL---
I believe this is identical to:
IBM 2315 TYPE
Part #     703
Description High Density: 100 TPI, 2200 BPI   
Sectors 12
--------------

The label on the disk seems to me to be saying I am correct.
I have spent an hour, diligently searching the web, trying to find information about this disk. I find a lot about the 8 sector LOW denisty, but nothing about this HIGH density disk.
What I am getting at is this:
High Density: 100 TPI, 2200 BPI700     Low Density: 100 TPI, 1100 BPI     8
701     High Density: 100 TPI, 2200 BPI     8
703         12

Part Density  TPI BPI   Sectors
700   Low     100 1100  8 
701   High    100 2200  8 
700   High    100 2200  12
The web site
http://www.ibm1130.net/functional/DiskStorage.html
says these disks held 1 megabyte. Assuming that is the capacity of the "Part 700; low density; 8 sector disk," then doubling the BPI would double the capacity to 2 megabytes. But what do the additional sectors do to the capacity. Do they just make data transfer faster, without changing capacity?

Any help would be appreciated.

[Carl Claunch]

The 2315 disk pack had eight sectors, not 12. The hardware logic in the 1131 (and in the 2310 external drives) implement a divide by two circuit, presenting these are four sectors per rotation to the rest of the logic and thus to the programmer. 

DEC and others licensed the technology and made similar, but often improved, drives. DEC had 12 sector physical packs for one of the PDP lines. Thus there are two kinds of 2315-like cartridges, with different sectoring. 

Using more sectors gives smaller capacity per sector, kind of obvious. The different word sizes of the two DEC systems led naturally to different divisions of the capacity of the cartridge.

DEC and others switched to the high density mode - recording more bits per inch by modulating the head at a higher frequency. DEC also used different positioning hardware, skipping the mechanical ratchet system of the 2310 that gives the 1130 disk access its characteristic buzz. 

The 2310 drive had 320 word sectors (640 byte-equivalent), four sectors per cylinder on each of the upper and lower surfaces. Thus 8 sectors x 320 words is 2,560 words or 5,120 bytes equivalent. 200 cylinders for user data, so 1MB equivalent which is 512,000 words. Actually a few extra cylinders to use as spares if there is a bad spot on the disk, plus one word at the front of each sector traditionally used to store the sector number. 

Carl