RCA 301, Programmer'S Reference Manual, Page: 38 / 235

progressing in the Record File Mode) then the Record File Mode instruction is transferred to the Record File Mode
(i e the contents of the NOR Register are transferred to FOR, the contents of the N Regtster are transferred to the
L Register, the contents of the A Register are transferred to the U Register, and the contents of the B
transferred to the V Register). The instruction is then executed in the Record File Mode with FOR, ,
Registers acting as the NOR, N, A and B Registers respectively. The instruction is then processed similarly to
normal instruction.
Immediately upon the transfer of an instruction from the Normal to the Record File Mode, the Normal Mode is
free to accept the next instruction.
If a Record File Mode instruction is staticized and the system is not equipped with a Record File Mode Control
Unit, the instruction will be skipped.
If a Record File Mode instruction is staticized and the Record File Mode is occupied, the Record File Mode
instruction will hold off in the Normal Mode until the Record File Mode is f ree; i.e., the Record File Mode Instruction
will occupy the Normal Mode, but will not be executed therein. When the Record File Mode becomes free, t e
instruction will be transferred to the Record File Mode for execution, freeing the Normal Mode for the next instruction.
Device Busy Hold-Off
As soon as an input/output instruction is staticized regardless of the mode in which it is to be executed, it
senses the input/output device it must use. If the device is busy, the instruction holds off in the Normal Mode until
the device is free.
Independent Operations
The following operations are independent operations:
Band selection
Track selection
Rewinding tapes
Paper advancing
Regardless of the mode in which the instructions causing these functions are executed, once any one of these
functions is initiated, it continues independent of the Computer. The instruction, then, is considered by the Computer
circuitry to have completed itself upon the initiation of the function, and the mode which the instruction occupied
is thereby freed for immediate use.
In a system equipped with both the Simultaneous Mode Control Unit and the Record File Mode Control Unit,
it is possible to have one instruction progressing in the Simultaneous Mode, another instruction progressing in the
Record File Mode, and still another instruction progressing in the Normal Mode and any number of independent
operations in progress, all simultaneously.
If a parity error occurs while a Record File Mode instruction is being executed the mode in which the error
occurred is stopped immediately and the instructions presently being executed in the other modes are completed before
the computer halts. If the error occurs in the Record File Mode, the FAL (Record File Alarm) light will be on as
•will
a nother error light which will indicate the type of error.
Time Sharing
Simultaneity in the RCA 301 System is accomplished through a technique known as "time sharing." All input/
output instructions need to access the High-Speed Memory for only a fraction of the time they are in progress. For
most of their execution times, input/output instructions must wait for some mechanical action in the input/output
device to occur (e.g., card movement, tape movement, print drum revolution, etc.).
III-8