CodeWarrior for MCU中的主题 Re: Byte or Bit Reference?

Byte or Bit Reference?

bpaehr — Fri, 09 Sep 2016 17:32:36 GMT

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

Re: Byte or Bit Reference?

ZhangJennie — Tue, 13 Sep 2016 05:39:01 GMT

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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Re: Byte or Bit Reference?

bpaehr — Tue, 13 Sep 2016 18:30:49 GMT

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

Re: Byte or Bit Reference?

ZhangJennie — Thu, 03 Nov 2016 04:06:13 GMT

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