
5-8
SYM53C876/876E Data Manual
SCSI SCRIPTS Instruction Set
I/O Instructions
SYM53C876 is operating in initiator or target
mode. Op Code selections 101-111 are con-
sidered Read/Write instructions, and are
described in that section.
Target Mode
Reselect Instruction
1. The SYM53C876 arbitrates for the SCSI bus
by asserting the SCSI ID stored in the SCID
register. If it loses arbitration, it tries again
during the next available arbitration cycle
without reporting any lost arbitration status.
2. If the SYM53C876 wins arbitration, it
attempts to reselect the SCSI device whose ID
is defined in the destination ID field of the
instruction. Once the SYM53C876 wins
arbitration, it fetches the next instruction from
the address pointed to by the DSP register.
This way the SCRIPTS can move on to the
next instruction before the reselection
completes. It continues executing SCRIPTS
until a SCRIPT that requires a response from
the initiator is encountered.
3. If the SYM53C876 is selected or reselected
before winning arbitration, it fetches the next
instruction from the address pointed to by the
32-bit jump address field stored in the DNAD
register. Manually set the SYM53C876 to
initiator mode if it is reselected, or to target
mode if it is selected.
Disconnect Instruction
The SYM53C876 disconnects from the SCSI bus
by deasserting all SCSI signal outputs.
Wait Select Instruction
1. If the SYM53C876 is selected, it fetches the
next instruction from the address pointed to
by the DSP register.
2. If reselected, the SYM53C876 fetches the
next instruction from the address pointed to
by the 32-bit jump address field stored in the
DNAD register. Manually set the
SYM53C876 to initiator mode when it is
reselected.
3. If the CPU sets the SIGP bit in the ISTAT
register, the SYM53C876 aborts the Wait
Select instruction and fetches the next
instruction from the address pointed to by the
32-bit jump address field stored in the DNAD
register.
Set Instruction
When the SACK/ or SATN/ bits are set, the cor-
responding bits in the SOCL register are set. Do
not set SACK/ or SATN/ except for testing pur-
poses. When the target bit is set, the correspond-
ing bit in the SCNTL0 register is also set. When
the carry bit is set, the corresponding bit in the
Arithmetic Logic Unit (ALU) is set.
Note: None of the signals are set on the SCSI
bus in target mode.
Clear Instruction
When the SACK/ or SATN/ bits are set, the cor-
responding bits are cleared in the SOCL register.
Do not set SACK/ or SATN/ except for testing
purposes. When the target bit is set, the corre-
sponding bit in the SCNTL0 register is cleared.
When the carry bit is set, the corresponding bit in
the ALU is cleared.
Note: None of the signals are reset on the SCSI
bus in target mode.
OPC2
OPC1
OPC0
Instruction Defined
0
0
0
0
1
0
0
1
1
0
0
1
0
1
0
Reselect
Disconnect
Wait Select
Set
Clear