28.2.9 __builtin_clr_prefetch

Description

Clear an accumulator and prefetch data ready for a future MAC operation.

xptr may be null to signify no X prefetch to be performed, in which case the values of xincr and xval are ignored, but required.

yptr may be null to signify no Y prefetch to be performed, in which case the values of yincr and yval are ignored, but required.

xval and yval nominate the address of a C variable where the prefetched value will be stored.

xincr and yincr may be the literal values: -6, -4, -2, 0, 2, 4, 6 or an integer value.

If AWB is non null, the other accumulator will be written back into the referenced variable.

For example:

 volatile register int16_t result asm("A");
 volatile register int16_t B asm("B");
 int16_t x_memory_buffer[256] __attribute__((space(xmemory)));
 int16_t y_memory_buffer[256] __attribute__((space(ymemory)));
 int16_t *xmemory;
 int16_t *ymemory;
 int16_t awb;
 int16_t xVal, yVal;

 xmemory = x_memory_buffer;
 ymemory = y_memory_buffer;
 result = __builtin_clr(&xmemory, &xVal, 2,
 &ymemory, &yVal, 2, &awb, B);

might generate:

 clr A, [w8]+=2, w4, [w10]+=2, w5, w13

The compiler may need to spill w13 to ensure that it is available for the write-back. It may be recommended to users that the register be claimed for this purpose.

After this instruction:

result will be cleared

xVal will contain x_memory_buffer[0]

yVal will contain y_memory_buffer[0]

xmemory and ymemory will be incremented by 2, ready for the next mac operation

Prototype

int16_t __builtin_clr_prefetch(
 int16_t **xptr, int16_t *xval, int16_t xincr,
 int16_t **yptr, int16_t *yval, int16_t yincr, 
 int16_t *AWB, int16_t AWB_accum);

Arguments

xptr – Integer pointer to x prefetch

xval – Integer value of x prefetch

xincr – Integer increment value of x prefetch

yptr – Integer pointer to y prefetch

yval – Integer value of y prefetch

yincr – Integer increment value of y prefetch

AWB – Accumulator write back location

AWB_accum – Accumulator to write back

Return Value

Returns the cleared value result to an accumulator.

Machine Instruction