TAL Programmer's Guide

Appendix B Managing Addressing
096254 Tandem Computers Incorporated B–1
This appendix discusses:
Extended pointer format
Accessing the upper 32K-word area of the user data segment
Creating and accessing an extended data segment
Accessing a code segment
The coding practices covered in this appendix predate automatic extended segment
management and are described here in support of existing programs.
Extended
Pointer Format
You can use an extended pointer to access the current user data segment, the system
data segment, or the current user code segment. You must use an extended pointer to
access an extended data segment.
Figure B-1 shows the format of extended pointers.
Figure B-1. Format of Extended Pointer
0
1
2
3
4
5
6
7
8
9
10
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12
13
14
15
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18
19
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21
22
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25
26
27
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30
31
High-order address word Low-order address word
A B Segment Page Word Byte
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Table B-1 explains the meanings of the bits in shown in Figure B-1.
Table B-1. Extended Pointer Format
Bits Meaning Values
<0> Absolute Mode specifier (A) 0 for nonprivileged use; 1 for privileged use
<1> Reserved (B) 0
<2:14> Segment specifier 0:1027 (relative extended address)
0:8191 (absolute extended address)
<15:20> Page specifier 0:63
<21:30> Word specifier 0:1023
<31> Byte specifier 0 in left byte; 1 in right byte