Assembler HCS12 how does register with index work with TST-instruction?

Question

Hello this is my test code:

        LDX #$2000
        LDY #$1000
        
        
        LDD #$0000
        
  la:   ADDD #1
        MOVB 1, X+, 1, Y+
        TST -1, X         <-- what do I check here?
        BNE la

My question is what do I check with the TST -1,X? I know that TST checks if my register is 0 or negative. But what does -1, X mean?. When is this condition not true?

Answer 1

This is a copy function, possibly used for null terminated strings. X and Y are the 16 bit index registers used to hold addresses.

With MOVB 1, X+, 1, Y+ you first move the data from address X to Y and then post-increment both addresses. TST -1, X checks the contents of the address that X had before that post-increment. If not zero then loop, otherwise stop.