RCA 301, Programmer'S Reference Manual

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These registers assume the function of the NOR, N, A and B Registers respectively.
If an operation is executed in the Record File Mode, the following registers are utilized:
Record File Operation Register (FOR) (receives OP Code)
L Register (receives N character)
U Register (receives A address)
V Register (receives B address)
These registers also assume the function of the NOR, N, A, and B Registers.
The D Register has a capacity of two RCA 301 Characters. Comparisons and arithmetic instructions utilize
this register.
The Bus Adder is used to modify the contents of the various address registers. The Bus Adder can increment or
decrement the contents of these registers by either 1 or 2, or leave them unchanged, thus permitting register contents
to be directly related to the currently processed characters or diads.
The A-B Equality Circuit is located between the A and B Registers and is used to compare their contents. When
they are equal, a flip-flop is set indicating to the Computer that the instruction-defined sector has been processed
and the instruction can be terminated. In instructions that specify the left and right boundaries of the HSM sector to
be processed, the contents of the A Register are always increased toward A-B equality and the contents of the B
Register remain unchanged.
The Memory Address Bus is a four character pathway between the Interchange, Computer Registers, and Input-
Output buffers. The particular pathways used are determined primarily by the Interchange.
The Previous Result Indicators, or PRI's are a set of flip-flops which preserve the sign of the result (or the zero
result) of an arithmetic operation for automatic reference by a subsequent decision instruction. If the result is
positive, the Previous Result Positive (PRP) flip-flop is set; if negative, the Previous Result Negative (PRN) flip-
flop is set; if zero, the Previous Result Zero (PRZ) flip-flop is set. The PRI's are set in most of the arithmetic and
logical instructions, and in certain search and transfer operations.
The PRI's may be sensed by use of the Conditional Transfer of Control Instruction.
Arithmetic Functions
The arithmetic function of addition and subtraction in the RCA 301 System is performed by the use of arithmetic
tables permanently stored in the High Speed Memory (Standard Memory Locations 0000-0199). These locations can be
written into only if a special button on the console is depressed, thereby preventing the accidental destruction of
these tables. The add and subtract instructions perform arithmetic operations utilizing a table lookup technique
where the arguments are the specified operands.
Automatic Storage of Final Contents of the A Register (STA)
STA is an automatic operation which occurs at the conclusion of selected RCA 301 instructions. In STA, the
final contents of the A Register are automatically stored in the High Speed Memory locations 0212-0215. This permits
the subsequent use of the final A Register contents. It is also a highly convenient programming technique which can
be utilized to eliminate memory searching time.
STA is performed in the following list of instructions:
Locate Symbol Left (*-
Locate Symbol Right
Transfer Data by Symbol Left &
Transfer Data by Symbol Right
Tape Read Forward Normal "4
Tape Read Reverse Normal fe
Block Read Record Normal
Sector Read Disc Normal
F
To preserve the final A Register contents of all other instructions, the Store Register instruction can be used.
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