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S3C2440A RISC MICROPROCESSOR ARM INSTRUCTION SET
3-47
SINGLE DATA SWAP (SWP)
31 19 15
Cond
28 16 11122123
B
20
00 Rn Rd
[3:0] Source Register
[15:12] Destination Register
[19:16] Base Register
[22] Byte/Word Bit
0 = Swap word quantity
1 = Swap word quantity
[31:28] Condition Field
22
00010 0000 Rm1001
27 87 43 0
Figure 3-23. Swap Instruction
The instruction is only executed if the condition is true. The various conditions are defined in Table 3-2. The
instruction encoding is shown in Figure 3-23.
The data swap instruction is used to swap a byte or word quantity between a register and external memory. This
instruction is implemented as a memory read followed by a memory write which are “locked” together (the
processor cannot be interrupted until both operations have completed, and the memory manager is warned to treat
them as inseparable). This class of instruction is particularly useful for implementing software semaphores.
The swap address is determined by the contents of the base register (Rn). The processor first reads the contents
of the swap address. Then it writes the contents of the source register (Rm) to the swap address, and stores the
old memory contents in the destination register (Rd). The same register may be specified as both the source and
destination.
The LOCK output goes HIGH for the duration of the read and write operations to signal to the external memory
manager that they are locked together, and should be allowed to complete without interruption. This is important in
multi-processor systems where the swap instruction is the only indivisible instruction which may be used to
implement semaphores; control of the memory must not be removed from a processor while it is performing a
locked operation.
BYTES AND WORDS
This instruction class may be used to swap a byte (B=1) or a word (B=0) between an ARM920T register and
memory. The SWP instruction is implemented as a LDR followed by a STR and the action of these is as described
in the section on single data transfers. In particular, the description of Big and Little Endian configuration applies to
the SWP instruction.