Programming instructions

and the instruction part (bits O-4) of this word are placed in the IR.
The C(PC) are then incremented by one.
If a two-cycle instruction is fetched, the following control state will be
either Defer or Execute. If a one-cycle instruction is fetched, the
operations specified will be performed during the last part of the
Fetch cycle. The next state will be Fetch.
DEFER: When bit 4 of a memory referehce instruction is a 1, the Defer
state is entered to perform the indirect addressing. The process of
indirect addressing is often referred to as deferring, in the sense that
access to the operand is deferred once to another memory cell. This
is why the primary control state in which this operation is performed
is called Defer. Bit 4 of a memory reference instruction is referred to
interchangeably as the Indirect or the Defer Bit.
EXECUTE: This state is established only when a memory reference
instruction is being performed. The contents of the memory cell
addressed are brought into the MB, and the operation specified by
the C(IR) is performed.
BREAK: When this state is established, the sequence of instructions
is broken for a Data Interrupt or a Program Interrupt. In both cases,
the break occurs only at the completion of the current instruction.
The Data Interrupt allows information to be transferred between
memory and an external device; when this transfer has been com-
pleted, the program sequence is resumed from the point of the break.
The Program Interrupt causes the sequence to be altered. The C(PC)
and the C(L) are stored in location 0000 and the program continues
from location 0001.
Instructions
The instruction code is specified by bits O-3 of a word. There are two types
of instructions: Memory Reference and Augmented.
MEMORY REFERENCE INSTRUCTIONS
The bit assignment of the memory reference instruction is shown in Figure
5. Bits O-3 determine the operation to be performed. Bits 5-17 specify the
address of the memory cell containing the operand. If bit 4 is a 1, then
indirect addressing occurs. In the following discussion, i is the mnemonic
symbol used to indicate indirect addressing.
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