Cray CRAY-1 - Scalar Functional Units; Scalar Add Unit; Scalar Shift Unit

SCALAR FUNCTIONAL UNITS
The scalar functional units perform operations on 64-bit operands obtained
from S registers and in most cases deliver the 64-bit results to an S
register. The exception is the population/leading zero count unit which
delivers its 7-bit result to an A register.
Four functional units are exclusively associated with scalar operations
and are described here. Three functional units are used for both scalar
and vector operations and are described under the section entitled
Floating Point Functional Units.
Scalar
add unit
The scalar add unit performs 64-bit integer addition and subtraction. It
implements instructions 060 and 061. The addition and subtraction are per-
formed in a similar manner. However, the two's complement subtraction
for the
061 instruction occurs as follows. The one's complement of the Sk
operand is added to the Sj operand. Then a one is added in the low order
bit position of the result.
No overflow is detected in the unit.
The functional unit time is three clock periods.
Scalar
shift unit
The scalar shift unit shifts the entire 64-bit contents of an S register
or shifts the double 128-bit contents of two concatenated S registers.
Shift counts are obtained from an A register or from the jk portion of
the
instruction. Shifts are end off with zero fill. For a double shift,
a circular shift is effected if the shift count does not exceed 64 and
the i and j designators are equal and non-zero.
The scalar shift unit implements instructions 052 through 057. Single
register shift instructions, 052 through 055, are executed in two clock
periods. Double-register
shift instructions, 056 and 057, are executed
in three clock periods.
2240004
3-12
E