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Instruction Set of the I/O Processor
Transfer Control Instructions
6-18
SYM53C875/875E Data Manual
Return Instruction
1. T he SYM53C875 can do a true/false
comparison of the ALU carry bit, or compare
the phase and/or data as defined by the Phase
Compare, Data Compare, and True/False bit
fields. If the comparisons are true, then the
SYM53C875 loads the DSP register with the
contents of the DSPS register. T hat address
value becomes the address of the next
instruction.
When a Return instruction is executed, the
value stored in the T EMP register is returned
to the DSP register. T he SYM53C875 does
not check to see whether the Call instruction
has already been executed. It will not generate
an interrupt if a Return instruction is executed
without previously executing a Call instruc-
tion.
2. If the comparisons are false, then the
SYM53C875 fetches the next instruction from
the address pointed to by the DSP register and
the instruction pointer will not be modified.
Interrupt Instructions
Interrupt
a) T he SYM53C875 can do a true/false
comparison of the ALU carry bit, or
compare the phase and/or data as defined
by the Phase Compare, Data Compare,
and True/False bit fields. If the
comparisons are true, then the
SYM53C875 generates an interrupt by
asserting the IRQ/ signal.
b) T he 32-bit address field stored in the
DSPS register (not DNAD as in 53C700)
can contain a unique interrupt service
vector. When servicing the interrupt, this
unique status code allows the ISR to
quickly identify the point at which the
interrupt occurred.
c) T he SYM53C875 halts and the DSP
register must be written to start any further
operation.
Interrupt on-the-Fly
a) T he SYM53C875 can do a true/false
comparison of the ALU carry bit or
compare the phase and/or data as defined
by the Phase Compare, Data Compare,
and True/False bit fields. If the
comparisons are true, and the Interrupt on
the Fly bit is set (bit 20), the SYM53C875
will assert the Interrupt on the Fly bit
(ISTAT bit 2).
Bits 26-24 SCSI Phase
T his 3-bit field corresponds to the three SCSI
bus phase signals which are compared with the
phase lines latched when SREQ/ is asserted.
Comparisons can be performed to determine
the SCSI phase actually being driven on the
SCSI bus. T he following table describes the
possible combinations and their correspond-
ing SCSI phase. T hese bits are only valid when
the SYM53C875 is operating in initiator
mode; when the SYM53C875 is operating in
the target mode, these bits should be cleared.
Bit 23 Relative Addressing Mode
When this bit is set, the 24-bit signed value in
the DSPS register is used as a relative offset
from the current DSP address (which is point-
ing to the next instruction, not the one cur-
rently executing). Relative mode does not
apply to Return and Interrupt SCRIPT S.
MSG
C/D
I/O
SCSI Phase
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Data out
Data in
Command
Status
Reserved out
Reserved in
Message out
Message in