LSI 53C875A technical manual Read-Modify-Write Cycles, A60 Register Address A60 2216

A[6:0]	Register Address - A[6:0]	[22:16]
	It is possible to change register values from SCRIPTS in
	read-modify-write cycles or move to/from SFBR cycles.
	A[6:0] selects an 8-bit source/destination register within
	the LSI53C875A.

ImmD		Immediate Data			[15:8]
		This 8-bit value is used as a second operand in logical
		and arithmetic functions.
A7		Upper Register Address Line [A7]			7
		This bit is used to access registers 0x80–0xFF.
R		Reserved			[6:0]
5.5.2 Second Dword
31					0
		DMA SCRIPTS Pointer Save (DSPS) Register
x x x x x x x x	x x x x x x x x		x x x x x x x x	x x x x x x x x
		Destination Address			[31:0]
		This field contains the 32-bit destination address where
		the data is to move.

5.5.3 Read-Modify-Write Cycles

During these cycles the register is read, the selected operation is performed, and the result is written back to the source register.

The Add operation is used to increment or decrement register values (or memory values if used in conjunction with a Memory-to-Register Move operation) for use as loop counters.

Subtraction is not available when 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.

Read/Write Instructions 5-23