Programming instructions

ACCUMULATOR (AC): Arithmetic operations are performed in this
18-bit register. The AC may be cleared and complemented. Its con-
tents may be rotated right or left with the Link. The contents of the
Memory Buffer may be added to the contents of the AC with the result
left in the AC. The contents of both these registers may be combined
by the logical operations AND and Exclusive OR, the result remaining
in the AC. The Inclusive OR may be formed between the AC and the
Accumulator Switches on the Operator Console (see below), and the
result left in the AC.
The Accumulator also acts as an input-output register. Under normal
operation all information transfers between core memory and an
external device must pass through the Accumulator.
LINK (L): This is a one-bit register used to extend the arithmetic
facility of the Accumulator. In l’s complement arithmetic, the Link
is an overflow indicator; in 2’s complement it functions as a carry
register. The Link may be cleared and complemented and its state
sensed independent of the AC. It is included with the AC in rotate
operations.
MEMORY BUFFER (MB): All information transferred between Core
Memory and the AC, Instruction Register, or Program Counter passes
through the MB. Information is read from a memory cell into the MB
and rewritten into the cell in one cycle time (8 microseconds). In-
structions are brought from memory into the MB to be decoded. The
MB serves also as a buffer for information transferred between Core
Memory and an external device in a Data Interrupt. The contents of
the MB may be incremented by one.
MEMORY ADDRESS REGISTER (MA): The address of the Core Memory
cell currently being accessed is contained in the 13-bit MA. Informa-
tion may enter the MA from the MB, Program Counter, or from an
external device operating in a Data interrupt.
INSTRUCTION REGISTER (IR): This is a 4-bit register which contains
the operation code of the instruction currently being performed by
the computer. Information enters the IR from the MB.
PROGRAM COUNTER (PC): The program sequence, that is, the order
in which instructions are performed, is determined by the PC. This
13-bit register contains the address of the memory cell from which
the next instruction will be taken. Information may enter the PC from
the MB, MA, or the Address Switches of the Operator Console.
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