Avago Technologies LSI53C876E - Manual (Page 215)

Read/Write Instructions 5-25
Subtraction is not available when SCSI First Byte Received (SFBR) is
used instead of data8 in the instruction syntax. To subtract one value
from another when using SFBR, first XOR the value to subtract
(subtrahend) with 0xFF, and add 1 to the resulting value. This creates the
2’s complement of the subtrahend. The two values are then added to
obtain the difference.
5.5.4 Move To/From SFBR Cycles
All operations are read-modify-writes. However, two registers are
involved, one of which is always the
SCSI First Byte Received (SFBR) .
Table 5.2 shows the possible functions of this instruction.
•
Write one byte (value contained within the SCRIPTS instruction) into
any chip register.
•
Move to/from the SCSI First Byte Received (SFBR) from/to any other
register.
•
Alter the value of a register with AND, OR, ADD, XOR, SHIFT LEFT,
or SHIFT RIGHT operators.
•
After moving values to the SCSI First Byte Received (SFBR) , the
compare and jump, call, or similar instructions are used to check the
value.
•
A Move-to-SFBR followed by a Move-from-SFBR is used to perform
a register to register move.
Table 5.2 Read/Write Instructions
Operator
Opcode 111
Read-Modify-Write Opcode 110
Move to SFBR Opcode 101
Move from SFBR
000 Move data into register.
Syntax: “Move data8 to
RegA” Move data into SCSI First
Byte Received (SFBR)
register. Syntax: “Move
data8 to SFBR”
Move data into register.
Syntax: “Move data8 to
RegA”
001
1
Shift register one bit to the
left and place the result in
the same register. Syntax:
“Move RegA SHL RegA” Shift register one bit to the
left and place the result in
the
SCSI First Byte
Received (SFBR) register.
Syntax: “Move RegA SHL
SFBR”
Shift the SCSI First Byte
Received (SFBR) register
one bit to the left and place
the result in the register.
Syntax: “Move SFBR SHL
RegA”