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S3C2410A ARM INSTRUCTION SET
3-7
BRANCH AND BRANCH WITH LINK (B, BL)
The instruction is only executed if the condition is true. The various conditions are defined Table 3-2. The instruction
encoding is shown in Figure 3-3, below.
31 2427
Cond Offset
28 23
[24] Link bit
0 = Branch 1 = Branch with link
[31:28] Condition Field
25
101 L
0
Figure 3-3. Branch Instructions
Branch instructions contain a signed 2's complement 24-bit offset. This is shifted left two bits, sign extended to 32
bits, and added to the PC. The instruction can therefore specify a branch of +/- 32Mbytes. The branch offset must
take account of the prefetch operation, which causes the PC to be 2 words (8 bytes) ahead of the current instruction.
Branches beyond +/- 32Mbytes must use an offset or absolute destination which has been previously loaded into a
register. In this case the PC should be manually saved in R14 if a Branch with Link type operation is required.
THE LINK BIT
Branch with Link (BL) writes the old PC into the link register (R14) of the current bank. The PC value written into R14
is adjusted to allow for the prefetch, and contains the address of the instruction following the branch and link
instruction. Note that the CPSR is not saved with the PC and R14[1:0] are always cleared.
To return from a routine called by Branch with Link use MOV PC,R14 if the link register is still valid or LDM
Rn!,{..PC} if the link register has been saved onto a stack pointed to by Rn.
INSTRUCTION CYCLE TIMES
Branch and Branch with Link instructions take 2S + 1N incremental cycles, where S and N are defined as sequential
(S-cycle) and internal (I-cycle).