ARM.SoC.Architecture

Medium range unconditional branches to goto sections

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Unformatted text preview: (offset shifted left 1 place); LR := oldPC + 3. Here 'oldPC' is the address of the second instruction; the return address has two bytes added to point to the next instruction and the bottom bit set to indicate that the caller is a Thumb routine. Format 3a is an alternative second step which gives the BLX variant; it is only available in architecture v5T. It uses the same first step as BL above: 1. (BL, H=0) LR := PC + (sign-extended offset shifted left 12 places); 2. (BLX) PC := LR + (offset shifted left 1 place) & Oxfffffffc; LR := oldPC + 3; the Thumb bit is cleared. Note that as the target is an ARM instruction, the offset only requires 10 bits and the computed PC value may still have bit [1] set, so it is cleared implicitly. The fourth format maps directly onto the ARM B{L}X instructions (see Section 5.5 on page 115, except that with BLX (available in architecture v5T only) r14 is set to the address of the following instruction plus 1, indicating that the caller was Thumb code). Here 'H' can be set to select a 'Hi' register (r8 to r15). Assembler format B<cond> <label> ; format 1 B BL <label> ; <label> ; format 2 format 3 - Thumb target - Thumb target - Thumb target BLX <label> ; B{L}X Rm ; format 3a - ARM target format 4 - ARM or Thumb target A branch and link generates both format 3 instructions. It is not intended that format 3 instructions are used individually; they should always appear in pairs. Likewise, BLX generates a format 3 instruction and a format 3a instruction. The assembler will compute the relevant offset to insert into the instruction from the current instruction address, the address of the target label, and a small correction for the pipeline behaviour. If the target is out of range an error message will be output. Equivalent ARM instruction Although formats 1 to 3 are very similar to the ARM branch and branch-with-link instructions, the ARM instructions support only word (4-byte) offsets whereas the Thumb instructions require...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.

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