platforms/boards/pyboard/assembler label branch

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

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) compare regA with regB (regA+regB), put the result into CPSR flags doesn't this turn the branch comparisons upside down? if so it's "gnarly"

tst(regA,regB) compare regA with regB (regA&regB), put the result into CPSR flags

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 (>=)

bgt(LABEL) branch if greater than (>)

blt(LABEL) branch if less than (<)

ble(LABEL) branch if less than or equal to (<=)

bcs(LABEL) branch if ??? no clue ???

bcc(LABEL) branch if ??? no clue ???

bmi(LABEL) branch if ??? no clue ???

bpl(LABEL) branch if ??? no clue ???

bvs(LABEL) branch if ??? no clue ???

bvc(LABEL) branch if ??? no clue ???

bhi(LABEL) branch if ??? no clue ???

bls(LABEL) branch if ??? no clue ???

label, branch and 'branch on logic' instructions

The Pyboard

comparison instructions

branch_if