The fixed-point instruction set performs binary arith metic on operands serving as addresses, index quanti
ties, and counts, as well as fixed-point data. In general,
both operands are signed and 32 bits long. Negative
quantities are held in two's-complement form.One operand is always in one of the 16 general registers;
the other operand may be in main storage or in a
general register.
The instruction set provides for loading, adding,
subtracting, comparing, multiplying, dividing, and
storing, as well as for the sign control, radix conver
sion, and shifting of fixed-point operands. The entire
instruction set is included in the standard instruction
set.
The condition code is set as a result of allsign control, add, subtract, compare, and shift operations.
Data Format
Fixed-point numbers occupy a fixed-length format
consisting of a one-bit sign followed by the integer
field. When held in one of the general registers, a
fixed-point quantity has a 31-bit integer field andoc cupies all 32 bits of the register. Some multiply, divide,
and shift operations use an operand consisting of 64
bits with a 63-bit integer field. These operands are
located in a pair of adjacent general registers and are
addressed by an even address referring to theleft most register of the pair. The sign-bit position of the
rightmost register contains part of the integer. Inreg ister-to-register operations the same register may be
specified for both operand locations.
Fullword Fixed-Point Number151 Integer o 1 31
Halfword Fixed-Point NumberI s\ Integer o 1 15
Fixed-point data in main storage occupy a 32-bit word
or a 16-bit halfword, with a binary integer field of 31
or 15 bits, respectively. The conversion instructions
Fixed-Point Arithmetic
use a 64-bit decimal field. These data must be located
on integral storage boundaries for these units ofinfor mation, that is, double-word, fullword, or halfword
operands must bc addressed with three, two, or one
low-ordcr address bit ( s) set to zero.
A halfword operand in main storage is extended to
a fullword as the operand is fetched from storage.
Subsequently, the operand participates as a full word
operand.
Number Representation
All fixed-point operands are treated as signed integers.Positive numbers are represented in true binary nota tion with the sign bit set to zero. Negative numbers
are represented in two's-complement notation with a
one in the sign bit. The two's complement of anum ber is obtained by inverting each bit of the number
and adding a one in the low-order bit position.
This type of number representation can be consider
ed the low-order portion of an infinitely long represen
tation of the number. When the number is positive, all
bits to the left of the most significant bit of thenum ber, including the sign bit, are zeros. When the num ber is negative, all these bits, including the sIgn bit,
are ones. Therefore, when an operand must beex tended with high-order bits, the expansion is achieved
by prefixing a field in which each bit is set equal to
the high-order bit of the operand.
Two's-complement notation does not include anega tive zero. It has a number range in which the set of
negative numbers is one larger than the set of positive
numbers. The maximum positive number consists of
an all-one integer field with a sign bit of zero, whereas
the maximum negative number consists of an all-zero
integer field with a one-bit for sign.
TheCPU cannot represent the complement of the
maximum negative number. When an operation, such
as a subtraction from zero, produces the complement
of the maximum negative number, the number remains
unchanged, and afixed-pOint overflow exception is
recognized. An overflow does not result, however,
when the number is complemented and the finalre suIt is within the representable range. An example of
this case is a subtraction from minus one. The product
of two maximum negative numbers is representable as
a double-length positive number.
Fixed-Point Arithmetic 23
ties, and counts, as well as fixed-point data. In general,
both operands are signed and 32 bits long. Negative
quantities are held in two's-complement form.
the other operand may be in main storage or in a
general register.
The instruction set provides for loading, adding,
subtracting, comparing, multiplying, dividing, and
storing, as well as for the sign control, radix conver
sion, and shifting of fixed-point operands. The entire
instruction set is included in the standard instruction
set.
The condition code is set as a result of all
Data Format
Fixed-point numbers occupy a fixed-length format
consisting of a one-bit sign followed by the integer
field. When held in one of the general registers, a
fixed-point quantity has a 31-bit integer field and
and shift operations use an operand consisting of 64
bits with a 63-bit integer field. These operands are
located in a pair of adjacent general registers and are
addressed by an even address referring to the
rightmost register contains part of the integer. In
specified for both operand locations.
Fullword Fixed-Point Number
Halfword Fixed-Point Number
Fixed-point data in main storage occupy a 32-bit word
or a 16-bit halfword, with a binary integer field of 31
or 15 bits, respectively. The conversion instructions
Fixed-Point Arithmetic
use a 64-bit decimal field. These data must be located
on integral storage boundaries for these units of
operands must bc addressed with three, two, or one
low-ordcr address bit ( s) set to zero.
A halfword operand in main storage is extended to
a fullword as the operand is fetched from storage.
Subsequently, the operand participates as a full word
operand.
Number Representation
All fixed-point operands are treated as signed integers.
are represented in two's-complement notation with a
one in the sign bit. The two's complement of a
and adding a one in the low-order bit position.
This type of number representation can be consider
ed the low-order portion of an infinitely long represen
tation of the number. When the number is positive, all
bits to the left of the most significant bit of the
are ones. Therefore, when an operand must be
by prefixing a field in which each bit is set equal to
the high-order bit of the operand.
Two's-complement notation does not include a
negative numbers is one larger than the set of positive
numbers. The maximum positive number consists of
an all-one integer field with a sign bit of zero, whereas
the maximum negative number consists of an all-zero
integer field with a one-bit for sign.
The
maximum negative number. When an operation, such
as a subtraction from zero, produces the complement
of the maximum negative number, the number remains
unchanged, and a
recognized. An overflow does not result, however,
when the number is complemented and the final
this case is a subtraction from minus one. The product
of two maximum negative numbers is representable as
a double-length positive number.
Fixed-Point Arithmetic 23