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5-30 SCSI SCRIPTS Instruction Set
signed (2’s complement), the jump can be forward or
backward.
A relative transfer can be to any address within a
16 Mbyte segment. The program counter is combined
with the 24-bit signed offset (using addition or
subtraction) to form the new execution address.
SCRIPTS programs may contain a mixture of direct
jumps and relative jumps to provide maximum versatility
when writing SCRIPTS. For example, major sections of
code can be accessed with far calls using the 32-bit
physical address, then local labels can be called using
relative transfers. If a SCRIPT is written using only
relative transfers it does not require any run time
alteration of physical addresses, and can be stored in and
executed from a PROM.
R Reserved 22
CT Carry Test 21
When this bit is set, decisions based on the ALU carry bit
can be made. True/False comparisons are legal, but Data
Compare and Phase Compare are illegal.
IF Interrupt-on-the-Fly 20
When this bit is set, the interrupt instruction does not halt
the SCRIPTS processor. Once the interrupt occurs, the
Interrupt-on-the-Fly bit (Interrupt Status One (ISTAT1)
bit 2) is asserted.
JMP Jump If True/False 19
This bit determines whether the LSI53C875A branches
when a comparison is true or when a comparison is false.
This bit applies to phase compares, data compares, and
carry tests. If both the Phase Compare and Data
Compare bits are set, then both compares must be true
to branch on a true condition. Both compares must be
falsetobranchonafalsecondition.