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Unformatted text preview: cache and data cache has been added. External memory and peripherals are accessed via an AMBA bus master unit, and a write buffer and memory management (ARM71OT and 720T) unit or memory protection (ARM740T) unit are also incorporated. The organization of the ARM710T and ARM720T CPUs is similar and is illustrated in Figure 12.1. ARM710T cache Since the ARM7TDMI processor core has a single memory port it is logical for it to be paired with a unified instruction and data cache. The ARM710T incorporates such a cache, with a capacity of 8 Kbytes. The cache is organized with 16-byte lines and is 4-way set associative. A random replacement algorithm selects which of the Figure 12.1 ARM710T and ARM720T organization. The ARM710T, ARM720T and ARM740T 319 tag RAM I tag RAM tag RAM Figure 12.2 The ARM71OT cache organization. four possible locations will be overwritten by new data on a cache miss. The cache uses a write-through strategy as the target clock rate is only a few times higher than the rate at which standard off-chip memory devices can cycle. The organization of the ARM710T cache is illustrated in Figure 12.2. Bits [10:4] of the virtual address are used to index into each of the four tag stores. The tags contain bits [31:11] of the virtual addresses of the corresponding data, so these tags are compared with bits [31:11] of the current virtual address. If one of the tags matches, the cache has hit and the corresponding line can be accessed from the data RAM using the same index (bits [10:4] of the virtual address) together with two bits which encode the number of the tag store which produced the matching tag. Virtual address bits [3:2] select the word from the line and, if a byte or half-word access is requested, bits [1:0] select the byte or half-word from the word. It is interesting to compare the ARM710T cache organization with that of the ARM3 and ARM610 (described in detail in Section 10.4 on page 279) since they illustrate the sorts of issues that arise in designing a cache for both good performance and low-power operation. Although there is, as yet, no final word on the...
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- Spring '09