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processed without any special indication of crossing
the maximum-address boundary.
Information Formats
Information is transmitted between storage and the
CPU or a channel one byte,
0"
a group of bytes, at
a time. Unless otherwise specified, a group of
bytes in storage is addressed by the leftmost byte of
the group. The number of bytes in the group is
either implied or explicitly specified by the
operation to be performed. When used in a CPU
operation, a group of bytes is called a field.
Within each group of bytes, bits are numbered in
a left-to-right sequence. The leftmost bits are
sometimes referred to as the "high-order" bits and
the rightmost bits as the "low-order" bits. Bit
numbers are not storage addresses, however. Only
bytes can be addressed. To operate on individual
bits of a byte in storage, it is necessary to access
the entire byte.
The bits in a byte are numbered 0 through 7,
from left to right.
The bits in an address are numbered 8 through
31. Within any other fixed-length format of
multiple bytes, the bits making up the format are
consecutively numbered starting from O.
For purposes of error detection, and in some
models for correction, one or more check bits may
be transmitted with each byte or with a group of
bytes. Such check bits are generated automatically
by the machine and cannot be directly controlled
by the program. References in this publication to
the length of data fields and registers exclude
mention of the associated check bits. All storage
capacities are expressed in nUl!lber of !Jytes.
When the length of an operand field is implied
by the operation code of an instruction, the field is
said to have a fixed length, which can be one, two,
3-2
IBM 4300 Processors Principles of Operation
four, or eight bytes.
When the length of an operand field is not
implied but is stated explicitly, the field is said to
have variable length. Variable-length operands can
vary in length by increments of one byte.
When information is placed in storage, the
contents of only those byte locations are replaced
that are included in the designated field, even
though the width of the physical path to storage
may be greater than the length of the field being
stored.
Integral Boundaries
Certain units of information must be located in
storage on an integral boundary. A boundary is
called integral for a unit of information when its
storage address is a multiple of the length of the
unit in bytes. Special names are given to fields of
two, four, and eight bytes when they are located on
an integral boundary. A halfword is a group of two
consecutive bytes on a two-byte boundary and is
the basic building block of instructions. A word is
a group of four consecutive bytes on a four-byte
boundary. A doubleword is a group of eight
consecutive bytes on an eight-byte boundary. (See
the figure "Integral Boundaries with Storage
Addresses. ")
When storage addresses designate halfwords,
words, and double words on integral boundaries, the
binary representation of the address contains one,
two, or three rightmost zero bits, respectively.
Instructions must appear on two-byte integral
boundaries, and channel-command words and the
storage operands of certain instructions must
appear on other integral boundaries. The storage
operands of most instructions do not have
boundary-alignment requirements.
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