91 lines
2.8 KiB
ReStructuredText
91 lines
2.8 KiB
ReStructuredText
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Comparison instructions
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=======================
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These perform an arithmetic or logical instruction on two arguments, discarding the result
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but setting the condition flags. Typically these are used to test data values without changing
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them prior to executing a conditional branch.
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Document conventions
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--------------------
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Notation: ``Rd, Rm, Rn`` denote ARM registers R0-R7. ``imm8`` denotes an immediate
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value having a width of 8 bits.
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The Application Program Status Register (APSR)
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----------------------------------------------
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This contains four bits which are tested by the conditional branch instructions. Typically a
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conditional branch will test multiple bits, for example ``bge(LABEL)``. The meaning of
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condition codes can depend on whether the operands of an arithmetic instruction are viewed as
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signed or unsigned integers. Thus ``bhi(LABEL)`` assumes unsigned numbers were processed while
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``bgt(LABEL)`` assumes signed operands.
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APSR Bits
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---------
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* Z (zero)
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This is set if the result of an operation is zero or the operands of a comparison are equal.
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* N (negative)
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Set if the result is negative.
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* C (carry)
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An addition sets the carry flag when the result overflows out of the MSB, for example adding
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0x80000000 and 0x80000000. By the nature of two's complement arithmetic this behaviour is reversed
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on subtraction, with a borrow indicated by the carry bit being clear. Thus 0x10 - 0x01 is executed
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as 0x10 + 0xffffffff which will set the carry bit.
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* V (overflow)
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The overflow flag is set if the result, viewed as a two's compliment number, has the "wrong" sign
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in relation to the operands. For example adding 1 to 0x7fffffff will set the overflow bit because
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the result (0x8000000), viewed as a two's complement integer, is negative. Note that in this instance
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the carry bit is not set.
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Comparison instructions
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-----------------------
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These set the APSR (Application Program Status Register) N (negative), Z (zero), C (carry) and V
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(overflow) flags.
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* cmp(Rn, imm8) ``Rn - imm8``
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* cmp(Rn, Rm) ``Rn - Rm``
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* cmn(Rn, Rm) ``Rn + Rm``
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* tst(Rn, Rm) ``Rn & Rm``
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Conditional execution
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---------------------
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The ``it`` and ``ite`` instructions provide a means of conditionally executing from one to four subsequent
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instructions without the need for a label.
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* it(<condition>) If then
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Execute the next instruction if <condition> is true:
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::
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cmp(r0, r1)
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it(eq)
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mov(r0, 100) # runs if r0 == r1
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# execution continues here
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* ite(<condition>) If then else
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If <condtion> is true, execute the next instruction, otherwise execute the
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subsequent one. Thus:
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::
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cmp(r0, r1)
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ite(eq)
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mov(r0, 100) # runs if r0 == r1
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mov(r0, 200) # runs if r0 != r1
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# execution continues here
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This may be extended to control the execution of upto four subsequent instructions: it[x[y[z]]]
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where x,y,z=t/e; e.g. itt, itee, itete, ittte, itttt, iteee, etc.
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