HASP_diskspace_management.txt

HASP strives to make DASD storage allocation very efficiently, not only needing no access to DASD to allocate
or de-allocate space, but also doing a good job of minimizing arm movement on moveable-head devices.    

HASP requires the use of entire volumes for SPOOLing input/output.  The management of this storage works as follows: 

A master cylinder bit-map is maintained in HASP.  This is a string of byes, in which each bit represents 1 cylinder 
on the SPOOL disk.  A one-bit represents a free cylinder, while a zero-bit shows that the give cylinder is allocated 
to some JOB.  

HASP also remembers for each disk which cylinder was the last referenced, thus always noting the current position 
of the read/write heads. 

Two key bit-maps exist for each JOB, one for SYSIN data and the other for SYSOUT data. Whenever a cylinder is required 
for a JOB, HASP searches for a free one in the following fashion:   

-It first searches the master bit-map for a free cylinder at the current position of any read/write head, i.e. where 
it can read or write without moving a head. 

-It then searches for a free cylinder at +1 from the current head position, then –1 from each, followed by +2, -2, etc.,
up to +8, -8 cylinders away from the current head position. 

If the above fails, it searches sequentially through all cylinders in the master bit-map.  When a cylinder is found, 
its bit is turned off (to zero) in the master bit-map, and turned on in the appropriate JOB bit-amp.  
The overall effect of this process is to minimize head movement.    

When disk storage for a JOB is to be released, the de-allocation scheme is extremely fast and efficient:   

The JOB’s bit-maps are just ORed into the master bit-map, thus returning all of the cylinders to free storage.