The basic structure of a System/360 consists of main
storage, a central processing unit(cPu), the selector
and multiplexor channels, and the input! output de
vices attached to the channels through control units.
It is possible for systems to communicate with each
other by means of sharedI/O devices, a channel, or
shared storage. Figure 1 shows the basic organization
of a single system.
MainStorage Storage units may be either physically integrated with
theCPU or constructed as stand-alone units. The stor
age cycle is not directly related to the internal cycling
of theCPU, thus permitting selection of optimum stor
age speed for a given word size. The physical differ
ences in the various main-storage units do not affect
the logical structure of the system.
Fetching and storing of data by theCPU are not af
fected by any concurrentI/O data transfer. If an I/O operation refers to the same storage location as the CPU operation, the accesses are granted in the se
quence in which they are requested. If the first refer
ence changes the contents of the location, any sub
sequent storage fetches obtain the new contents. Con
currentI/O and CPU references to the same storage
location never cause a machine-check indication.
Main
Storage
-Control '-- System Structure
Information Formats
The system transmits information between main stor
age and theCPU in units of eight bits, or a multiple
of eight bits at a time. An eight-bit unit of information
is called a byte, the basic building block of all formats.
A ninth bit, the parity or check bit, is transmitted
with each byte and carries parity on the bytes. The
parity bit cannot be affected by the program; its only
effect is to cause an interruption when a parity error
is detected. References to the size of data fields and
registers, therefore, exclude the associated parity bits.
All storage capacities are expressed in number of bytes
provided, regardless of the physical word size actually
used.
Bytes may be handled separately or grouped to
gether in fields. A halfwordis a group of two consecu
tive bytes and is the basic building block of instruc
tions. A word is a group of four consecutive bytes; a
double word is a field consisting of two words (Figure
2). The location of any field or group of bytes is spe
cified by the address of its leftmost byte.
The length of fields is either implied by the oper
ation to be performed or stated explicitly as part of
the instruction. When the length is implied, the in
formation is said to have a fixed length, which can be
either one, two, four, or eight bytes.
When the length of a field is not implied by theInput/ I 1 - Output c Unit c f---t - Device ....c: U f-----t c-- .... f---i Q) 1 f-----t ""5 f---i ::E f----i Central Processing t- Input/ Unit Control t-- I-- Output f------\ Unit t- Device W c f---t t-- C c f---t ....c: U --I .... .8 --I u Q) --I W Vl --I Figure 1. IBM System/360 Basic Logical Structure
System Structure 7
storage, a central processing unit
and multiplexor channels, and the input! output de
vices attached to the channels through control units.
It is possible for systems to communicate with each
other by means of shared
shared storage. Figure 1 shows the basic organization
of a single system.
Main
the
age cycle is not directly related to the internal cycling
of the
age speed for a given word size. The physical differ
ences in the various main-storage units do not affect
the logical structure of the system.
Fetching and storing of data by the
fected by any concurrent
quence in which they are requested. If the first refer
ence changes the contents of the location, any sub
sequent storage fetches obtain the new contents. Con
current
location never cause a machine-check indication.
Main
Storage
-
Information Formats
The system transmits information between main stor
age and the
of eight bits at a time. An eight-bit unit of information
is called a byte, the basic building block of all formats.
A ninth bit, the parity or check bit, is transmitted
with each byte and carries parity on the bytes. The
parity bit cannot be affected by the program; its only
effect is to cause an interruption when a parity error
is detected. References to the size of data fields and
registers, therefore, exclude the associated parity bits.
All storage capacities are expressed in number of bytes
provided, regardless of the physical word size actually
used.
Bytes may be handled separately or grouped to
gether in fields. A halfword
tive bytes and is the basic building block of instruc
tions. A word is a group of four consecutive bytes; a
double word is a field consisting of two words (Figure
2). The location of any field or group of bytes is spe
cified by the address of its leftmost byte.
The length of fields is either implied by the oper
ation to be performed or stated explicitly as part of
the instruction. When the length is implied, the in
formation is said to have a fixed length, which can be
either one, two, four, or eight bytes.
When the length of a field is not implied by the
System Structure 7