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Instruction Set of the I/O Processor
I/O Instructions
6-12
SYM53C825A/825AE Data Manual
Table Relative
Treats the alternate jump address as a relative
jump and fetches the device ID, synchronous off-
set, and synchronous period indirectly. Adds the
value in bits 23-0 of the first four bytes of the
SCRIPT S instruction to the data structure base
address to form the fetch address.
Bit 24
Select with AT N/
T his bit specifies whether SAT N/ will be
asserted during the selection phase when the
SYM53C825A is executing a Select instruc-
tion. When operating in initiator mode, set this
bit for the Select instruction. If this bit is set on
any other I/O instruction, an illegal instruction
interrupt is generated.
Bit 23-20 Reserved
Bits 19-16 E ncoded SCSI Destination ID
T his 4-bit field specifies the destination SCSI
ID for an I/O instruction.
Bits 15-11Reserved
Bit 10
Set/Clear Carry
T his bit is used in conjunction with a Set or
Clear instruction to set or clear the Carry bit.
Setting this bit with a Set instruction asserts
the Carry bit in the ALU. Setting this bit with a
Clear instruction deasserts the Carry bit in the
ALU.
Bits 8-7 Reserved
Bit 9
Set/Clear Target Mode
T his bit is used in conjunction with a Set or
Clear instruction to set or clear target mode.
Setting this bit with a Set instruction config-
ures the SYM53C825A as a target device (this
sets bit 0 of the SCNT L0 register). Setting this
bit with a Clear instruction configures the
SYM53C825A as an initiator device (this
clears bit 0 of the SCNT L0 register).
Bit 6
Bit 3
Set/Clear SACK /
Set/Clear SAT N/
T hese two bits are used in conjunction with a
Set or Clear instruction to assert or deassert
the corresponding SCSI control signal. Bit 6
controls the SCSI SACK / signal; bit
3 controls
the SCSI SAT N/ signal.
Setting either of these bits will set or reset the
corresponding bit in the SOCL register,
depending on the instruction used. T he Set
instruction is used to assert SACK / and/or
SAT N/ on the SCSI bus. T he Clear instruction
is used to deassert SACK / and/or SAT N/ on
the SCSI bus.
Since SACK / and SAT N/ are initiator signals,
they will not be asserted on the SCSI bus
unless the SYM53C825A is operating as an
initiator or the SCSI Loopback Enable bit is
set in the ST EST 2 register.
T he Set/Clear SCSI ACK /AT N instruction
would be used after message phase Block Move
operations to give the initiator the opportunity
to assert attention before acknowledging the
last message byte. For example, if the initiator
wishes to reject a message, an Assert SCSI
AT N instruction would be issued before a
Clear SCSI ACK instruction.
Bits 2-0 Reserved
Second Dword
Bits 31-0Start Address
T his 32-bit field contains the memory address
to fetch the next instruction if the selection or
reselection fails.
If relative or table relative addressing is used,
this value is a 24-bit signed offset relative to the
current DSP register value.
Command
Table Offset
Alternate Jump Offset