
Instruction Set of the I/O Processor
Transfer Control Instructions
6-20
SYM53C810A Data Manual
or Call instruction. Because it is signed (twos
compliment), the jump can be forward or
backward.
A relative transfer can be to any address within
a 16-MB segment. T he program counter is
combined with the 24-bit signed offset (using
addition or subtraction) to form the new exe-
cution address.
SCRIPT S programs may contain a mixture of
direct jumps and relative jumps to provide
maximum versatility when writing SCRIPT S.
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 S instruction
uses only relative transfers it would not require
any run time alteration of physical addresses,
and could be stored in and executed from a
PROM.
Bit 21
Carry Test
When this bit is set, decisions based on the
ALU carry bit can be made. True/False com-
parisons are legal, but Data Compare and
Phase Compare are illegal.
Bit 20
Interrupt on the Fly
When this bit is set, the Interrupt instruction
will not halt the SCRIPT S processor. Once the
interrupt occurs, the Interrupt on the Fly bit
(ISTAT bit 2) will be asserted.
Bit 19
Jump If True/False
T his bit determines whether the
SYM53C810A should branch when a compar-
ison is true or when a comparison is false. T his
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 false to branch on a
false condition.
Bit 18
Compare Data
When this bit is set, the first byte received from
the SCSI data bus (contained in SFBR regis-
ter) is compared with the Data to be Com-
pared Field in the Transfer Control instruction.
T he Wait for Valid Phase bit controls when this
compare will occur. T he Jump if True/False bit
determines the condition (true or false) to
branch on.
Bit 17
Compare Phase
When the SYM53C810A is in initiator mode,
this bit controls phase compare operations.
When this bit is set, the SCSI phase signals
(latched by SREQ/) are compared to the Phase
Field in the Transfer Control instruction; if
they match, the comparison is true. T he Wait
for Valid Phase bit controls when the compare
will occur. When the SYM53C810A is operat-
ing in target mode this bit, when set, tests for
an active SCSI SAT N/ signal.
Bit 16
Wait For Valid Phase
If the Wait for Valid Phase bit is set, the
SYM53C810A waits for a previously unser-
viced phase before comparing the SCSI phase
and data. If the Wait for Valid Phase bit is clear,
the SYM53C810A compares the SCSI phase
and data immediately.
Bit 19
Result of
Compare
Action
0
0
1
1
False
True
False
True
Jump Taken
No Jump
No Jump
Jump Taken