Testing status flags

4x5n · 2011-12-27 17:03

I've got a bunch of SX micro-controllers and am looking into writing code for a simple first project. There are no sensors that require advance mathematics. I'm going through the instruction set looking for instructions for testing flag states in the status register. In particular I'm interested in the DC flag. However I don't see any test instructions for flags or individual bits in a register. Is there such a command or am I going to have to mask off the bit with an "and" and then act on the zero flag?

Masking off the bit and acting on it wouldn't be the worst thing in the world but I'd rather not if there's an instruction that will do so directly.

Zoot · 2011-12-27 17:17

You can use JNB, JB, SB and SNB in assembly. In SX/B you can just use the bit name (and/or byte name and bit position) in an if/then statement. E.G.,

:doit
ADD fileRegister, W
:testdcbit
SB DC
   MOV W, #0 ' set W to 0 if DC = 0
SNB DC
   MOV W, #1 ' set W to 1 if DC = 0
JB DC, :label1
  MOV fileRegister, #$FF
:label1
JNB DC, :label2
  MOV fileRegister, #$80
:label2
' alternatively you could always use a byte.bitindex:

JNB fileRegister.7, :label3 ' only jump if bit7 = 0
  MOV fileRegister, #$80
:label3

IF DC = 0 THEN
   ' do some SX/B stuff
ELSE
  ' do some SX/B stuff
ENDIF

Obviously some very special status bits have dedicated instructions, e.g., SNC, SC, JC, JNC, SZ, SNZ, JZ, JNZ. But you could still use the same bit tests, e.g.

JNB Z, :somelabel

But it's not as efficient.

4x5n · 2011-12-27 18:07

Wow, that was fast and exactly what I was looking for. I figured there had to be a single instruction that would be able to check the DC flag!

Looks like I need to spend more time going over the instruction list for the SX!

Zoot · 2011-12-27 20:23

Well, SNB is a single word instruction and JNB/JB as well as JC/JNC and JZ/JNZ are all two word instructions (test/address), so in the end it works out to be the same, really.

At least the instruction set just isn't that big

Here is a really useful cheat sheet.

4x5n · 2011-12-29 16:38

The only chips I've done any assembly programming on were the old 8bit Intel (8085, z80) and Motorola chips and that was a very long time ago. Now I'm trying to learn to program in Spin, Pasm as well as SX/b and sasm!

Trying to keep things straight is more then a slight challenge. On the positive side I'm having a lot of fun. Yes I know I should at least stick with one processor but I got a bunch of SX processors at a price that I couldn't resist. Now all I have to do is come up with ways to use them. :-)

Testing status flags

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