label, branch and 'branch on logic' instructions

Label's and the branch (and branch_if ... branch on result of a comparison) functions are the equivalent of 'goto' in other simple languages ... they are used to implement programme logic.

label(LABEL) - target of a branch or conditional branch (branch_if - bxx function), LABEL is of your own devising.

b(LABEL) - branch to (goto a) named label

Example;

@micropython.asm_thumb
def test():
  mov(r0, 42)       #put 42 in register 0
  b(END)            #branch to label END
  mov(r0, 666)      #put 666 in register 0
  label(END)
>>> test()
42
r0 never got set to 666, because we skipped past that operation to go to **label(END)***

'branch on logic' (aka if aka 'make a decision') is a 2 step operation;

first; comparison (register to register, register to value)
then; check the results of the comparison and branch_if something is the case

To properly understand the use of the comparison instructions, it should be noted that most of the exposed ARM Thumb instructions set the condition flags. Aside from the explicit cmp instruction, the arithmetic instructions will set the condition codes depending on whether the result is greater than, equal to, or less than zero. More surprisingly from those coming from other assemblers, the mov instructions will also set the condition codes depending on the value deposited in the target register. Thus the code fragment below returns 0 regardless of its arguments unless the commented instruction is removed (when it returns 1 if the arguments differ, otherwise the equal value passed in the two registers).

@micropython.asm_thumb
def test(r0, r1):
    sub(r3, r0, r1)
    mov(r0, 0) # sets eq
    beq(DONE)
    mov(r0, 1)
    label(DONE)

comparison instructions

cmp(reg, val) compare reg with val (reg-val), put the result into CPSR flags

cmp(regA, regB) compare regA with regB (regA-regB), put the result into CPSR flags

cmn(regA, regB) add regA to regB (regA+regB), discard the result but set the CPSR flags. Typical use: compare registers holding numbers with opposite signs. If their absolute values are equal the eq flag will be set.

tst(regA,regB) and regA with regB (regA&regB), discard the result but set the CPSR flags (i.e. multiple bit test). The behaviour of this may seem counterintuitive: the eq flag is set if the result of the and operation is zero. The behaviour is in fact consistent with that of other instructions.

CPSR flags are some magic place for holding the result of a comparison, that is 'inspected' by the bxx operations in the next step, the branch_if instruction.

branch_if

bgt(LABEL) branch if greater than

Example;

@micropython.asm_thumb
def greater_than_life(r0):
  cmp(r0, 42)
  bgt(GT)   #r0>42 goto END
  mov(r0, 0)    #r0 = 0 (False in uPy)
  b(END)    #goto END
  label(GT)
  mov(r0, 1)    #r0 = 1 (True in uPy)
  label(END)
>>>greater_than_life(15)
0
>>>greater_than_life(42)
0
>>>greater_than_life(43)
1

beq(LABEL) branch if equal

bne(LABEL) branch if not equal

bge(LABEL) branch if greater than or equal to (>=) operands treated as signed

bgt(LABEL) branch if greater than (>) operands treated as signed

blt(LABEL) branch if less than (<) operands treated as signed

ble(LABEL) branch if less than or equal to (<=) operands treated as signed

bcs(LABEL) branch if carry flag is set

bcc(LABEL) branch if carry flag is clear

bmi(LABEL) branch if negative

bpl(LABEL) branch if positive

bvs(LABEL) branch if overflow flag set

bvc(LABEL) branch if overflow flag is clear

bhi(LABEL) branch if higher (> using unsigned comparison)

bls(LABEL) branch if lower or equal (<= using unsigned comparison)

The overflow flag is set if the result, viewed as a two's compliment number, has the "wrong" sign in relation to the operands. For example adding 1 to 0x7fffffff will set the overflow bit because the result (0x8000000), viewed as a two's compliment integer, is negative. Note that in this instance the carry flag is not set: the result has overflowed into the most significant bit only.

An addition sets the carry flag when the result overflows out of the MSB, for example adding 0x80000000 and 0x80000000.

Long branches

The branch instructions have a limited field width for branch destinations (which are PC relative). For long programs where the assembler produces a "branch not in range" error this can be overcome with the "wide" variants such as

beq_w(LABEL) long branch if equal

if_then

Execute the following instruction if a condition is true

cmp(r0, r1)
it(eq)
mov(r0, 100) # happens only if r0 == r1

The condition codes listed above are supported.

if_then_else

If a condition is true execute the following instruction, otherwise execute the one after

cmp(r0, r1)
ite(eq)
mov(r0, 100) # runs if r0 == r1
mov(r0, 200) # runs if r0 != r1

subroutines (internal functions)

You can have subroutines (internal functions) within the body of an assembler function. The ARM processor uses r15 as the program counter and r14 as a link register. To call a subroutine the bl(label) instruction loads the link register with the return address and jumps to the passed label. To return (to the instruction after the bl call) the bx(lr) instruction loads the program counter with the link register contents

def f(r0):
    # jump over the internal functions
    b(entry)
    label(func1)
    add(r0, 2)
    bx(lr) # Return
    label(func2)
    sub(r0, 1)
    bx(lr)
    label(entry)
    bl(func1) # Subroutine call
    bl(func2)

Note that, for nested subroutines, you need to push and pop the link register.

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