1.11.3 DSP ALU Status Bits

The high byte of the STATUS Register is used by the DSP class of instructions and it is modified when data passes through one of the adders. It provides status information about overflow and saturation for both accumulators. The Saturate A, Saturate B, Overflow A and Overflow B (SA, SB, OA, OB) bits provide individual accumulator status, while the Saturate AB and Overflow AB (SAB, OAB) bits provide combined accumulator status. The SAB and OAB bits provide an efficient method for the software developer to check the register for saturation or overflow.

The OA and OB bits are used to indicate when an operation has generated an overflow into the Guard bits (bits 63 through 71) of the respective accumulator. This condition can only occur when the processor is in Super Saturation mode or if saturation is disabled. It indicates that the operation has generated a number which cannot be represented with the lower 62 bits of the accumulator.

The SA and SB bits are used to indicate when an operation has generated an overflow out of the MSb of the respective accumulator. The SA and SB bits are active, regardless of the Saturation mode (Disabled, Normal or Super) and may be considered “sticky.” Namely, once the SA or SB bit is set to ‘1’, it can only be cleared by software, regardless of subsequent DSP operations. When it is required, the BCLR instruction can be used to clear the SA or SB bit.

In addition, the SA and SB bits can be set by software, enabling efficient context state switching.

For convenience, the OA and OB bits are logically ORed together to form the OAB flag, and the SA and SB bits are logically ORed to form the SAB flag. These cumulative Status bits provide efficient overflow and saturation checking when an algorithm is implemented. Instead of interrogating the OA and OB bits independently for arithmetic overflows, a single check of OAB can be performed. Likewise, when checking for saturation, SAB may be examined instead of checking both the SA and SB bits. Note that clearing the SAB flag will clear both the SA and SB bits.