Ctl Bus Priority Number Assignments - HP 3000 III Series Manual

System/CPU Overview
I
CTL BUS
PRIORITY
PRIORITY PRIORITY
PRIORI
NO.0AND 1 NO.2 AND 3 .NO.5
NO.4
512 K WORDS 512 K WORDS
PORT
LOWER MEMORY MODULE
UPPER MEMORY MODULE
CPU/lOP
CONTROL·
BANKS0-7
BANKS 8-15
LER
TY
\.._-------
- - - - - - - - - )
y
MAIN MEMORY
Figure 2-2. CTL Bus Priority Number Assignments
can be transferred in and out of Main Memory on demand. (Code
consists of executable instructions and unchanging constants of a
program or subprogram. As the code is executed, the manipulated
values are referred to as data.) When a program is executed, only
those segments of code and data required at a particular time
actually reside in Main Memory and all other related segments
remain in secondary memory until they in turn are required. When
a particular code segment is no longer needed, it is overlayed in
Main Memory by the next required code segment. (Code segments
are non-modifiable and reentrant.) If a code segment is needed
again, it is again copied from the secondary memory disc where it
resides. Data segments, however, are dynamic and their contents
can be changed during the programs execution. Therefore, when a
particular data segment is no longer needed, it is copied back
into the secondary memory disc and replaces the original data
segment version. The vacated Main Memory space is then available
for other segments. This process of transferring segments be-
tween secondary memory and Main Memory is referred to as swapping
and permits large programs that actually exceed Main Memory's
word capacity to be executed concurrently and still allow Main
Memory space for additional user programs.
2-11. Variable-Length Segmentation
Variable-length segmentation of code and data is used to facili-
tate multiprogramming. It minimizes waste of memory resources
due to internal fragmentation and allows the operating system to
deal with logical rather than physical entities. This means that
a particular subprogram can be contained within one segment
rather than arbitrarily divided between two physical pages, thus
minimizing the amount of swapping that need be accomplished while
executing the subprogram. The location and size of all executing
code segments are maintained in a Code Segment Table and the lo-
cation and size of all associated data segments are maintained in
2-8