MACHINE INSTRUCTION FORMAT
The 70/25 instruction format is variable in length.
instruction may contain either two, four, or six
The first byte of each instruction is an operation code.
The format of the second byte varies from one in-
struction to the next.
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.
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:
Packing and Unpacking
Logical Operations (And, Or, Excl. Or)
Conditional and Unconditional Branch
Test Under Mask
Set P2 Register
These bits are not used (ignored) by the
Input/Output (Post Status)
True and False Exercise
Data Edited for display purposes may
be in packed format.
A numeric field in unpacked format is
assumed to contain an (F)16 in the high
order four bytes of each byte.
When packing or unpacking a field, the
rightmost byte has its zone and numeric