From 616db8c8929201c066bd2c561d8f4e679abba6aa Mon Sep 17 00:00:00 2001 From: moshix Date: Sun, 24 Jul 2022 15:17:08 -0500 Subject: [PATCH] Create HASP_diskspace_management.txt --- HASP_diskspace_management.txt | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) create mode 100644 HASP_diskspace_management.txt diff --git a/HASP_diskspace_management.txt b/HASP_diskspace_management.txt new file mode 100644 index 0000000..931d879 --- /dev/null +++ b/HASP_diskspace_management.txt @@ -0,0 +1,28 @@ +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.