GE 235 System Manual page 36

Bit positions 5 and 6 of an instruction word designate the index word to be used in modifying the
operand address portion of the instruction. Binary configurations select index words a s follows:
01
=
location 0001, 10
=
location 0002, and 11
=
location 0003. Bits 00 in positions 5 and 6 indicate
that no modification i s to be performed, If the additional 31 index word groups a r e available, an
instruction i s provided to allow the selection of any one of the 32 groups a t any point in the
program.
If an instruction in the I register calls f o r automatic address modification, an extra word time
(6 microseconds) i s required to accomplish the operation.
The sequence of events in an automatic address modification is:
The instruction to be executed i s read into the I register.
Bits 5 through 19 of thedesignatedindexword and bits 7 through 19 of the I register a r e added
together and the result replaces bits 5 through 19 of the I register. Bits 0 through 4 of the
I register a r e unchanged.
The modified instruction in the I register i s executed.
The original instruction in memory i s not changed.
INSTRUCTION REPERTOIRE
The descriptions on the following pages a r e those of instructions which may be completely executed
by the Central P r o c e s s o r without assistance from any of the other system units. Descriptions
associated with peripheral units may be obtainedelsewhere in the manual following the description
of each peripheral device. Instructions in this section have been grouped under the following
headings:
Data Transfer
Arithmetic
Shift
Branch
Address Modification
Auxiliary Arithmetic Unit
Real Time Clock
Each description gives the mnemonic operation code f o r the instruction and an indication of whether
a memory location (Y) o r a constant (K)
is
required in the operand address portion. Some instruc-
tions do not require an operand, s o f o r these no entry will be found under the Operand Address
column. The letter X under the Index column indicates that the instruction can be indexed. Exe-
cution times a r e given in microseconds. The execution times include the fetching of the instruction
and data.
In all instructions which involve the bringing of a word from memory, the word in memory remains
unchanged. For most instructions involving the transfer of information from registers, the
condition of the register after execution
is
unchanged.