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S3C2410A ARM INSTRUCTION SET
3-5
BRANCH AND EXCHANGE (BX)
This instruction is only executed if the condition is true. The various conditions are defined in Table 3-2.
This instruction performs a branch by copying the contents of a general register, Rn, into the program counter, PC.
The branch causes a pipeline flush and refill from the address specified by Rn. This instruction also permits the
instruction set to be exchanged. When the instruction is executed, the value of Rn[0] determines whether the
instruction stream will be decoded as ARM or THUMB instructions.
31 2427 19 15 8 7 0
00 0 1 10 0 0 11 1 1 11 1 1 11 1 1 00 0 1Cond Rn
28 16 111223 20 4 3
[3:0] Operand Register
If bit0 of Rn = 1, subsequent instructions decoded as THUMB instructions
If bit0 of Rn =0, subsequent instructions decoded as ARM instructions
[31:28] Condition Field
Figure 3-2. Branch and Exchange Instructions
INSTRUCTION CYCLE TIMES
The BX instruction takes 2S + 1N cycles to execute, where S and N are defined as sequential (S-cycle) and non-
sequential (N-cycle), respectively.
ASSEMBLER SYNTAX
BX - branch and exchange.
BX {cond} Rn
{cond} Two character condition mnemonic. See Table 3-2.
Rn is an expression evaluating to a valid register number.
USING R15 AS AN OPERAND
If R15 is used as an operand, the behavior is undefined.