provides an information format and control signal
sequences that are independent of the type of con­
trol unit and channel and provide a uniform means
of attaching and controlling various types of I/O devices. I/O devices that do not use the I/O interface
employ the same information format and control
signal sequences. Devices and Control Units
Input/ output devices include such equipment as card
readers and punches, magnetic tape units, disk stor­
age, drum storage, typewriter-keyboard devices,
printers, teleprocessing devices, and sensor-based
equipment.
Many I/O devices function with an external doc­
ument, such as a punched card or a reel of magnetic
tape. Some I/O devices handle only electrical sig­
nals, such as those found in sensor-based networks.
In either case, I/O device operation is regulated by a
control unit. The control-unit function may be
housed with the I/O device or in the CPU, or a sep-
18 System/370 Principles of Operation
arate control unit may be used. In all cases, the
control-unit function provides the logical and buffer­
ing capabilities necessary to operate the associated I/O device. From the programming point of view,
most control-unit functions merge with I/O device
functions.
System Console
The system console provides the functions necessary
to operate and control the system. It consists of a
system control panel and, in most cases, an associat­
ed console device, which may also be used as an I/O device for communicating with the supervisory pro­
gram and problem programs. The need for operator
manipulation of manual controls is held to a mini­
mum by the system design and the governing super­
visory program.
The main functions provided by the system con­
sole include power-on/ off, reset, initial-pro gram­
loading, start/stop, and display and enter functions.
Program Execution
Contents Instructions .
Operands. Instruction Format
Register Operands I mmediate Operands
Storage Operands
Address Generation .
Program Status Word I nstruction Execution
Branching Interruptions. Sequence of Storage References I nstruction Fetch
DAT Table Fetches . Key-in·Storage Accesses
Storage-Operand References
Storage-Operand Fetch References
Storage-Operand Store References
Storage-Operand Update References
Storage-Operand Consistency . Single-Access References . Block-Concu rrent References
Consistency Specification . Relation Between Operand Accesses Serial ization .
Normally, operation of the CPU is controlled by
instructions taken in sequence. This sequence is
governed by the program status word (PSW), which
contains the primary information required for proper
program execution. A change in the sequential opera­
tion may be caused by branching, LOAD PSW, in­
terruptions, or manual intervention.
Instructions
Each instruction consists of two major parts: (1) an
operation code, which specifies the operation to be
performed, and (2) the designation of the operands
that participate. Operands Operands can be grouped in three classes: operands
located in registers, immediate operands, and oper­
ands in main storage. Operands may be either ex-
plicitly or implicitly designated.
Register operands can be located in general,
floating-point, or control registers, with the type of
register identified by the operation code. The regis­
ter containing the operand is specified by identifying
the register in a four-bit field, called the R field, in
the instruction. For some instructions an operand is · 19 · 19 ·20 ·20 ·20 ·20 · 21 ·22 .22
.22
.22
.23
.23
.24
.24
.25
.25
.25
.25
.26
.26
.27
.27
.27
.28
located in an implicitly designated register, the regis­
ter being implied by the operation code.
Immediate operands are contained within the
instruction, and the eight-bit field containing the
immediate operand is called the I field.
Operands in main storage may either have an
implied length, be specified by a bit mask, or, in
other cases, be specified by a four-bit or eight-bit
length specification, called the L field, in the instruc­
tion. The addresses of operands in main storage are
specified by means of a format that uses the con­
tents of a general register as part of the address. This
makes it possible to: Specify a complete address by using an abbrevi­
ated notation. Perform address manipulation using instructions
which employ general registers for operands. Modify addresses by program means without
alteration of the instruction stream. Operate independently of the location of data
areas by directly using addresses received from
other programs.
The address used to refer to main storage either is
contained in a register designated by the R field in
the instruction or is calculated from a base address,
Program Execution 19
Previous Page Next Page