title: Assembler HCS12 how does register with index work with TST-instruction?
body: <p>Hello this is my test code:</p>
  <pre><code>        LDX #$2000
          LDY #$1000
          
          
          LDD #$0000
          
    la:   ADDD #1
          MOVB 1, X+, 1, Y+
          TST -1, X         &lt;-- what do I check here?
          BNE la
  </code></pre>
  <p>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?</p>