Data Channel Command Word; Fundamental Components - IBM 7090 Instruction-Reference

when one operation is about finished and an instruction is needed. The author of the
program must provide the proper instruction word in storage and must build his program
so that the CPU has the address for that instruction when it is needed.
Each field of the instruction word has its particular significance because the CPU
brings the word in as an instruction. The sign position is always a part of the operation
code. Along with the sign, either the remainer of the prefix field or the decrement field
dictates what operation is to be done. If there is nothing in the remainder of the prefix,
the sign and decrement contain the operation code. If either of the two remaining posi-
tions of the prefix contains a bit, the prefix contains the entire operation code and the
decrement field is used for another purpose.
The address field usually contains the address of a data word in storage. This data
word is brought into the CPU as a part of whatever arithmetic or logic function is called
for by the operation code. Thus, the instruction not only dictates the operation to be
performed, but also specifies the address of the data to be used. In some instances, the
address field is a part of the operation code. When this is the case, the address field is
not used to address data in storage.
The tag field is a means of causing the CPU to calculate a storage address that is
different from the address field of the instruction.
2.2.03 Data Channel Command Word
Similar in format and application to the CPU instruction word, the data channel com-
mand word gains its special significance by being called out of storage by the control
function in the data channel. This, as in CPU, occurs when one operation is finished
and the data channel needs to be directed what to do next. The two major differences
are that:
1. The prefix is always the operation code in the data channel.
2. Positions 3-17 are a word counter. Whether the operation is reading or writing,
once the command is in channel, the word count becomes one less each time a
word is handled. Thus, when the word count becomes zero, the channel must
ask for a new command.
Data channel operations always start by unit selection from a CPU instruction.
Independent operation continues on the same unit as long as successive commands re-
quire it. Once the succession of commands is broken by a command to disconnect,
that unit and the channel that was operating have completed their assignment. This is
all planned in advance by the author of the program, and commands are stored where
the data channel will call for them at the proper time.
2.3.00 FUNDAMENTAL COMPONENTS
-The fundamental components described in this section are the operating features
(Figure 2.3-1) used in the operation A+B
=
C, print C. For the most part, these are
groups of binary storage positions where information may be placed outside of the stor-
age unit itself. These groups are commonly called registers.
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