Byte or Bit Reference?

[bpaehr]

In start08.c from my MC9S08SH8 Startup_Code folder, I found this code snippet (I've removed irrelevant comments and follow-on code):

static void Init(void) {

   int i;

   asm {

ZeroOut:

         LDA    _startupData.nofZeroOuts:1 ; /* nofZeroOuts */

        INCA

         STA    i:1 ; /* i is counter for number of zero outs */

         LDA    _startupData.nofZeroOuts:0 ; /* nofZeroOuts */

        INCA

        STA   i:0

         LDHX   _startupData.pZeroOut ; /* *pZeroOut */

        BRA   Zero_5

Zero_3:

.

.

}

What are the "i:0" and "i:1" references?  From the definition of "i" as an int, (a 16-bit, 2-byte value), they APPEAR to be low-byte and high-byte references, especially since the "STA" opcode is dealing with the 8-bit accumulator.  I can't see that there is any reference to a structure or bitfield since "i" is defined locally.

It appears that

   "STA  i:1"

means

      "store the entire 8-bit accumulator in the high byte of variable 'i' ".

It doesn't make any sense if I try to look at it from the individual bits.  Am I correct?

Bob

[ZhangJennie]

Hi,

regarding to "STA  i:1"

here, "i" is a variable name. "1" is offset address of "i".

let's take example "i" is at address 0x100. thus "STA  i:1" means stores the contents of A in memory address 0x101(0x100+1)

other examples:

"STA  i:2" : stores the contents of A in memory address 0x102

"STA  i:0" : stores the contents of A in memory address 0x100

"STA  i:5" : stores the contents of A in memory address 0x105

Have a great day,
Jennie Zhang

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[bpaehr]

Jennie:

Thank you for clarifying that.

I am curious, though...  using your example where "i" is at location 0x100, wouldn't

"STA  i+5"

also store the accumulator at address 0x105, which is the way I have always known how to store the Accumulator at a fixed offset from a given address location?

Is this "offset syntax" standard usage industry-wide, or only available on NXP/Freescale processors (used with CodeWarrior)?  I could not find any reference to this type of offset usage on the internet.

Bob

[ZhangJennie]

Hi,

instruction STA doesn't support syntax "STA  i+5".

I think the offset syntax is widely used in many MCU assembly code.

for more information about STA usage, see below. I extracted it from instruction set. see attached document.

Have a great day,
Jennie Zhang

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