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.

6.5.4 Move To/From SFBR Cycles

All operations are read-modify-writes. However, two registers are involved, one of which is always the SFBR. The possible functions of this instruction are:

Write one byte (value contained within the SCRIPTS instruction) into any chip register.

Move to/from the 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 SFBR, the compare and jump, call, or similar instructions may be used to check the value.

A Move-to-SFBR followed by a Move-from-SFBR is used to perform a register-to-register move.

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Instruction Set of the I/O Processor

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LSI 53C810A technical manual Move To/From Sfbr Cycles