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Machine Instruction Format; Exercise - RCA Spectra 70 Training Manual

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MACHINE INSTRUCTION FORMAT
The 70/25 instruction format is variable in length.
An
instruction may contain either two, four, or six
bytes.
The first byte of each instruction is an operation code.
The format of the second byte varies from one in-
struction to the next.
In
some instructions it is used
as a binary length counter (L). In others , the byte is
divided into two length counters of four bits each
(Ll' L2). In still others, it is used to hold a mask
(M), or one or more General Register numbers (R),
(R 1 -R 3 ). The second byte of a I/O command con-
tains a trunk and device designation.
The third and fourth bytes hold the address displace-
ment (D) and the number of the General Register (B)
which contains the base address to be associated
with that displacement.
3rd byte
4th byte
In a two-address instruction the fifth and sixth bytes
constitute the Band D field of the second address.
The machine formats and the type of instructions
using each format are shown below:
SIX-BYTE INSTRUCTIONS
Binary Arithmetic
Decimal Arithmetic
Decimal Comparison
Packing and Unpacking
Data Movement
Logical Operations (And, Or, Excl. Or)
Logical Comparison
Data Editing
Input/ Output
7
FOUR-BYTE INSTRUCTIONS
IGN:
lop 81M41IG~IB241D2
121
1
op
81
R:IIG~I
B241 D2
121
Conditional and Unconditional Branch
Test Under Mask
Set P2 Register
Load Multiple
Store Multiple
These bits are not used (ignored) by the
instruction.
TWO-BYTE INSTRUCTIONS
Halt
Input/Output (Post Status)
True and False Exercise
T
F
1.
Data Edited for display purposes may
be in packed format.
T
F
2.
A numeric field in unpacked format is
assumed to contain an (F)16 in the high
order four bytes of each byte.
T
F
3.
When packing or unpacking a field, the
rightmost byte has its zone and numeric
portions reversed.

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This manual is also suitable for:

Spectra 70/25