A PDF about the instruction set architecture of the ATI R700 series GPUs reveals the upcoming Radeon HD 4890 (RV790) will have at least one architectural difference with the Radeon HD 4870 (RV770):
Burst memory reads are not supported by the RV770; however, the 710, 730, 740, and 790 do support it. Chips after R770 support burst reads and memory-read instructions. This allows up to 16 consecutive locations to be read into up to 16 consecutive GPRs. This adds a new field BURST_CNT to MEM_RD_DWORD0.
For each iteration of the burst, the DST_GPR is incremented by 1 and the ARRAY_BASE is incremented by 1. The SRC_GPR is not affected.