A+B = C, Print C - IBM 7090 Instruction-Reference

The circuits in the multiplexor are actually switching circuits rather than registers.
These switching functions allow either the CPU or the data channel to use storage for
purposes of addressing and data transmission. The multiplexor also signals storage
whether the operation is to be stored or read out, governed by control from the CPU or
data channel.
The storage register accepts a word from storage for use by the CPU, or from the
CPU for forwarding to the memory data register. This is true for both instructions
and data. The storage register also plays a part in certain arithmetic and logic opera-
tions that are not used in A+B
=
C, print C.
The address register and program counter are the addressing registers of the CPU.
The address register, in every CPU operation involving storage, contains the storage
address. This address is sent to the memory address register when CPU is using stor-
age.
The program counter keeps account of the progress of the program. This counter
normally advances by one after each instruction has been called for. Thus, the normal
course of events is to call for instructions from ascending addresses in storage. When
one operation is complete, the current reading in the program counter goes to the ad-
dress register for forwarding to the memory address register. The author of the pro-
gram is responsible for having the next instruction stored at this address.
The adders and accumulator are the main arithmetic components of the entire sys-
tem. Although the adders are not registers and have no capacity for holding numbers,
they play such a prominent part in almost every CPU operation that they cannot be con-
sidered as an ordinary switching circuit.
The adders perform a merging function, having the capacity for operating on an
entire word at one time. Numbers from two sources can be added together and carries
simultaneously added in. Two full words can be sent to the adders, and the complete
answer can be taken almost immediately to a register. In addition, the adders provide
the only means of entry to the accumulator and the only direct route from the address
field of the storage register to the address register.
The major use of the accumulator is to receive the output of the adders. The name
accumulator, although not completely appropriate for a noncounting, nonadding register,
is applicable in that this register receives and holds sums from the adders.
The program register receives and holds the operation code of each CPU instruction.
Decoding the configuration of binary bits in the positions of this register establishes the
controls for the operation to be done. Each time that an operation is ended and a new
one is to begin, the old operation code is cleared from the program register and the
new code is set in. Thus, the controls for the operation completed are dropped and the
controls for the new operation are brought into effect.
2.4.00 A+B
=
C, PRINT C
The following explanation of 7090 's operation in performing A+B
=
C, print C is more
detailed than the general sequence in Section 1. 4.1. Review that section before proceeding.
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