Reference Guide

40 Chapter 3

HP-UX Architecture Conventions

Spaces

for the duration of program execution. The identiﬁer of the code space is

placed in space register 4 (%sr4) and the identiﬁer of the data space is

placed in space register 5 (%sr5).

When writing an assembly language program, declare a space named

$TEXT$ for executable code, and a space named $PRIVATE$ for

modiﬁable data. Constant data or literals that you do not plan to modify

during program execution, can be placed in either space. Placing

constant data in the $TEXT$ space decreases the size of the nonsharable

part of your program and improves the overall efﬁciency of the operating

system.

The particular space registers mentioned above play an important role in

virtual addressing. While many of the branching instructions, such as

BL, BLR, and BV, are capable of branching only within the currently

executing code space (called PC-space), two of the branching instructions,

BE and BLE, require that you specify a space register as well as an offset.

These instructions allow you to branch to code executing in a different

space. On HP-UX systems, normally all code for a program is contained

in one space, so all BE and BLE instructions should be coded to use %sr4.

In contrast, the memory reference instructions, such as LDW and STW,

allow a choice between two forms of addressing: long and short. With

long addressing, you can choose any of the space registers 1 through 3 for

the space identiﬁer part of the virtual address. The space offset is formed

as the sum of an immediate displacement and the contents of a general

through 7 is chosen automatically, based on the high-order two bits of the

base register. Each space addressed by these four space registers is

effectively divided into four quadrants, with a different quadrant of each

space accessible via short addressing.

On HP-UX systems, all of a program's code is placed in quadrant zero of

the $TEXT$ space, or %sr4, (space offsets from 0 through 0x3FFFFFFF).

The data is placed in quadrant one of the $PRIVATE$ space, or %sr5

(space offsets from 0x40000000 through 0x7FFFFFFF). Therefore,

literal data in the code space and modiﬁable data in the data space can

be addressed using the short addressing technique, without any concern

for the space registers.

The identiﬁer for shared memory segments, including shared library

text, is placed into space register 6 (%sr6). Shared memory and shared

library text are placed into quadrant two of the shared memory space

(offsets 0x80000000 through 0xBFFFFFFF). The identiﬁer for system