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The Processor and the Operating System 27
Figure 5 is a map of where these registers lie. In addition to these registers, the addresses 258
through 378 are also registers, but are not (except for a few isolated cases) used in assembly
programming.
A
B
P
R
R4
R5
R6
R7
(reserved)
"
~
(reserved)
I
I
Db _ /
Dmama
Dmac
C
D
Ar2
Se
(reserved)
I
Pa
I
Dmapa
address
o
1
2
3
4
5
6
7
10
11
12
13
14
15
16
17
20
21
22
23
24
25
37
Figure 5.
Map of Lowest Memory
All of these registers can be referenced either by their names or by their actual addresses. The
two methods are equivalent, though reference by name is recommended as a programming
practice.
In addition to the above internal registers, there are some "external" registers which reside in
the computer read/write memory. They are-
Name
Ar1
Base page
-
Oper_1
Oper_2
Result
Address
(octal)
177770-177773
177620-177701
177702
177703
I
177704
Description
BCD arithmetic accumulator
Base_page temporary area (50 words)
Arithmetic utility operand address registers
Arithmetic utility result address register
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