Cray CRAY-1 - Instruction Issue; Special Register Values

The unconditional branch to (Bjk) instruction requires only one parcel.
For this instruction, there is a 7-bit operation code (gh) followed by
a null i field and a combined jk field which specifies a B register that
contains a parcel address. The format is not illustrated.
SPECIAL REGISTER VALUES
The So and A 0 registers provide special values when referenced in the j
or k fields of an instruction. In these cases, the special value is used
as the operand and the actual value of the So or Ao register is ignored.
Such a use does not alter the actual value of the So or Ao register. If
So or Ao is used in the i field, the actual value of the register is
provided as the operand.
Field
Ai, i
= 0
Aj, j = 0
Ak, k = 0
Si, i = 0
Sj, j = 0
Sk, k = 0
Ah, h = 0
INSTRUCTION ISSUE
Operand value
(Ao)
o
1
(So)
o
2
63
o
Instructions are read a parcel at a time from the instruction buffers and
delivered to the NIP register. The instruction issues and is passed to
the
CIP register when the conditions in the functional unit and registers
are such that the functions required for execution may be performed with-
out conflicting with a previously issued
instruction. Instruction parcels
may issue at a maximum rate of one per clock period. Once an instruction
has been delivered to the CIP it is considered as issued and it must be
completed in a fixed time frame following its final clock period in the CIP
register. No delays Rre allowed from issue to delivery of data to the
destination operating
registers.
Entry to the NIP is blocked for the second half of a two-parcel instruction.
The parcel is delivered to the LIP register, instead. The blank NIP for
the
second parcel is issued as a do-nothing instruction in the CIP.
2240004
4-5
E