AT75C datasheet(13/171 Pages) ATMEL

13 / 171 page

AT75C DSP Subsystem

1368C–INTAP–08/02

when the value is transferred to the XDB. The saturation in move instructions can be

disabled by the SAT bit in the register ST0. When limiting occurs, the L flag in ST0 is

set. Refer to “Status Registers” on page 23 for more details.

The LIM instruction activates saturation on a 36-bit Ax-accumulator. When there is an

overflow from the high portion of an Ax-accumulator to the extension bits and a LIM

instruction is executed, the accumulator is limited to a full-scale, 32-bit positive (0 X

7FFFFFFF) or negative (0 X 8000000) value. Limiting will be correctly performed when

the value is operated on by the LIM instruction. The LIM instruction can use the same

accumulator for both source and destination or it can use one source Ax-accumulator,

which will not change, and transfer the limited result into the other Ax-accumulator. For

more details, refer to the LIM instruction in “Instruction Set” on page 31. When limiting

occurs, the L flag in ST0 is set. The SAT bit in ST0 has no effect on this instruction.

Refer to “Status Registers” on page 23 for more details.

Swapping the

Accumulators

The Ax-accumulators can be swapped with the Bx-accumulators in a single cycle. It is

possible to swap two 36-bit registers or all four 36-bits registers. Swapping is also

enabled between a specific Ax-accumulator into one of the Bx-accumulators, and in the

same cycle, from that Bx-accumulator into the other Ax-accumulator. Similarly, swap-

ping is enabled between a specific Bx-accumulator into one of the Ax-accumulators, and

in the same cycle, from that Ax-accumulator into the other Bx-accumulator. For a sum-

mary of the 14 swap options and other details, refer to the SWAP instruction in

“Instruction Set” on page 31.

Note that during interrupt context switching, the A1 and B1 accumulators are automati-

cally swapped. For more details, refer to “Interrupt Context Switching” on page 27.

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