11.9.3. Byte operations

A byte store (STRB) repeats the bottom 8 bits of the source register four times across data bus outputs 31 through 0. The external memory system should activate the appropriate byte subsystem to store the data.

Because ARM720T duplicates the byte to be written across the databus and internally rotates bytes after reading them from the databus, a 32-bit memory system only needs to have control logic to enable the appropriate byte. There is no need to rotate or shift the data externally.

To ensure that all of the databus is driven during a byte read, it is valid to read a word back from the memory.

Little-endian operation

A byte load (LDRB) expects the data on data bus inputs 7 through 0 if the supplied address is on a word boundary, on data bus inputs 15 through 8 if it is a word address plus one byte, and so on. The selected byte is placed in the bottom 8 bits of the destination register. The other 3 bytes on the databus are ignored. See Figure 11.5.

Big-endian operation

A byte load (LDRB) expects the data on data bus inputs 31 through 24 if the supplied address is on a word boundary, on data bus inputs 23 through 16 if it is a word address plus one byte, and so on. The selected byte is placed in the bottom 8 bits of the destination register. The other three bytes on the databus are ignored. See Figure 11.6.

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