Assembly Motorola 68k, how to branch if a tested bit is zero?

Question

I am writing a program in assembly using easy68k, I was looking for a way to make a branch if a tested bit is equal to zero [Z=0 in SR].

I searched a lot but didn't get an answer,

for example, I have this line:

  BTST #0,D2 ;Testing the LSB bit in data register 0

Now I want a way, or if there exists, an instruction to check Z in the SR, so I can know if the tested bit is zero or not.

Answer 1

The instruction to test condition codes and branch based on them is generically referred to as Bcc. The individual names for these conditional branch instructions are based on what they would mean if the immediately preceding instruction was CMP.

But, by themselves, all they do is test a condition code and take the branch. So, you can do:

BTST #0,D2 ;Testing the LSB bit in data register 0
BEQ LabelForZSet
;Code for Z clear

Answer 2

Branch on Z-Flag:

btst #0,d2
beq bitIsCleared
; or
bne bitIsSet

Z-Flag to register:

btst #0,d2
sne  d0
; d0 (byte only) will now be 0xFF if bit was set, 0x00 otherwise)
; if a word is required add ext.w d0
; if a long is required add also ext.l d0