This preview shows page 1. Sign up to view the full content.
Unformatted text preview: her than leave the datapath largely idle during the data transfer cycle, the ALU holds the address components from the first cycle and is available to compute an auto-indexing modification to the base register if this is required. (If auto-indexing is not required the computed value is not written back to the base register in the second cycle.) Data transfer instructions ARM instruction execution 83 Figure 4.5 Data processing instruction datapath activity. The datapath operation for the two cycles of a data store instruction (SIR) with an immediate offset are shown in Figure 4.6 on page 84. Note how the incremented PC value is stored in the register bank at the end of the first cycle so that the address register is free to accept the data transfer address for the second cycle, then at the end of the second cycle the PC is fed back to the address register to allow instruction prefetching to continue. It should, perhaps, be noted at this stage that the value sent to the address register in a cycle is the value used for the memory access in the following cycle. The address register is, in effect, a pipeline register between the processor datapath and the external memory. (The address register can produce the memory address for the next cycle a little before the end of the current cycle, moving responsibility for the pipeline delay out into the memory when this is desired. This can enable some memory devices to operate at higher performance, but this detail can be postponed for the time being. For now we will view the address register as a pipeline register to memory.) 84 ARM Organization and Implementation Figure 4.6 SIR (store register) datapath activity. When the instruction specifies the store of a byte data type, the 'data out' block extracts the bottom byte from the register and replicates it four times across the 32-bit data bus. External memory control logic can then use the bottom two bits of the address bus to activate the appropriate byte within the memory system. Load instructions follow a similar pattern except that the data from memory only gets as far as the 'data in' register on the second cycle and a third cycle is needed to t...
View Full Document
This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09