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SYM53C876/876E Data Manual
SCSI SCRIPTS Instruction Set
Read/Write Instructions
Clear instruction to set or clear target mode.
Setting this bit with a Set instruction config-
ures the SYM53C876 as a target device (this
sets bit 0 of the SCNTL0 register). Setting
this bit with a Clear instruction configures the
SYM53C876 as an initiator device (this clears
bit 0 of the SCNTL0 register).
Bits 8-7 Reserved
Bit 6
Set/Clear SACK/
Bits 5-4 Reserved
Bit 3
Set/Clear SATN/
These 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 con-
trols the SCSI SATN/ signal.
Setting either of these bits sets or resets the
corresponding bit in the SOCL register,
depending on the instruction used. The Set
instruction is used to assert SACK/ and/or
SATN/ on the SCSI bus. The Clear instruc-
tion is used to deassert SACK/ and/or SATN/
on the SCSI bus.
Since SACK/ and SATN/ are initiator signals,
they are not asserted on the SCSI bus unless
the SYM53C876 is operating as an initiator or
the SCSI Loopback Enable bit is set in the
STEST2 register.
The Set/Clear SCSI ACK/ATN instruction is
used after message phase Block Move opera-
tions 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, it issues an Assert
SCSI ATN instruction before a Clear SCSI
ACK instruction.
Bits 2-0 Reserved
Second Dword
Bits 31-0
Start Address
This 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.
Read/Write Instructions
The Read/Write instruction supports addition,
subtraction, and comparison of two separate val-
ues within the chip. It performs the desired opera-
tion on the specified register and the SFBR
register, then stores the result back to the speci-
fied register or the SFBR. If the COM bit
(DCNTL, bit 0) is cleared, Read/Write instruc-
tion can not be used.
First Dword
Bits 31-30 Instruction Type - Read/Write
Instruction
The Read/Write instruction uses operator bits
26 through 24 in conjunction with the op
code bits to determine which instruction is
currently selected.