Boxis R700 User Manual download pdf (Page 35)

ATI R700 Technology

Data Sharing 2-17

bank_offset = (thread_id >> 2) * dst_stride + dst_index

thread_id - SIMD_WAVE_REL mode controls:

0 : absolute – relative to threads within a wavefront.

1 : relative to the thread at the beginning of each group.

dst_stride - Destination stride from instruction for write to shared memory, in

dwords. Legal values: 4,8,12,16,...64.

dst_index - Destination index from instruction for write to shared memory in

dwords. Legal values: 4,8,12,16,...60.

The shader program provides addresses for reads, with a bank id and dword

bank offset. A maximum of four threads access the shared memory per clock

cycle in one of three addressing modes, based on mux_cntl field of

MEM_DSR_WORD1 (see the microcode description for MEM_DSR_WORD1, on

page 10-48).

• DSR_MUX_NONE enables each thread to address any aligned four-dword

entry of the shared memory.

• DSR_MUX_FFT_PERMUTE selection enables a 16 bank dword butterfly

read across the 512 bit output of the shared memory.

• DSR_MUX_WORD_SELECT enables any single dword read of the shared

memory.

If the compiler cannot determine that a read of four threads does not contain

bank conflicts, the instruction is repeated four times, with one of four successive

threads enabled on each pass to prevent conflicts. These instruction iterations

are forced if the waterfall bit is set in the instruction.

A special broadcast read mode can be enabled to do a fast read of one to four

dwords that can be returned either to all threads in a wavefront or to shared

registers. The broadcast read returns data to all GPRs within the wavefront in

four clocks cycles. This mode writes all threads, regardless of active or predicate

masks; the address to be read must be stored in the src GPR of the first thread

of the wavefront.

In SIMD_WAVE_REL absolute addressing mode, a clause can be used with one

set of writes, followed by a set of reads, without using any barrier synchronization

mechanism, as long as the exchanger operations are in one clause. The relative

modes require barrier synchronization.

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Boxis R700 User Manual Page 35