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C166 Var verses immediate as index value

Hi All, I'm having a real hard time coming up with a reason why using a local var as an index into a struct fails but that same line with an immediate value specified works properly. Here's what I mean..

void main(void)
	{
	unsigned char cTemp, cValue;

// Do whats required to get under way.
//----------------------------------------------------------------------------
 	MAIN_System_Init();


// Endless While loop that the micro runs forever (hopefully).
//----------------------------------------------------------------------------
	while(gcTRUE)
		{
		cValue = 0x0C;

		cTemp = *port[0x0C].lsr;
		cTemp = *port[0x0C].txrx;

		cTemp = *port[cValue].lsr;
		cTemp = *port[cValue].txrx;

		}
	}

The first two reads to cTemp work properly (with the value 0x0C specified) but the following two reads using the cValue variable as the index fail? BTW, The reason that I say they fail is that the reads should generate an external bus read to a uart register. The first two reads generate a chip select and the second two do not. I've tried changing compiler optimizations and options but I'm running out of ideas. I'm using ver 4.20c of the C compiler and version 4.20 of most everything else. Any suggestions would be really appreciated. Thanks. ~Tim

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0 Mik Kleshov over 23 years ago in reply to Tim Hutchinson

The first thing I would do is look at the generated code. I think it's much easier than looking at external bus accesses and gives much more information.
And another thing. Once I had this weird problem when display in my target worked in debugger mode (running off external RAM) and refused to work when running from internal flash. The reason was that the microcontroller does not release chip select between two accesses to external bus if it runs off internal flash. It's perfectly correct and documented. I had two insert dummy accesses two XRAM to release the chip select. Maybe that's the reason why you can't see enough transitions on chip select?
Regards,
- Mike
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0 Mik Kleshov over 23 years ago in reply to Tim Hutchinson

The first thing I would do is look at the generated code. I think it's much easier than looking at external bus accesses and gives much more information.
And another thing. Once I had this weird problem when display in my target worked in debugger mode (running off external RAM) and refused to work when running from internal flash. The reason was that the microcontroller does not release chip select between two accesses to external bus if it runs off internal flash. It's perfectly correct and documented. I had two insert dummy accesses two XRAM to release the chip select. Maybe that's the reason why you can't see enough transitions on chip select?
Regards,
- Mike
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0 Jon Ward over 23 years ago in reply to Mik Kleshov

I just compiled the following:

unsigned char lsr = 12;
unsigned char txrx = 34;

struct ct_st
  {
  unsigned char *lsr;
  unsigned char *txrx;
  };


volatile struct ct_st port [] =
  {
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
    { &lsr, &txrx },
  };


void main(void)
	{
	unsigned char  cTemp, cValue;

// Do whats required to get under way.
//----------------------------------------------------------------------------
// 	MAIN_System_Init();


// Endless While loop that the micro runs forever (hopefully).
//----------------------------------------------------------------------------
	while(1)
		{
		cValue = 0x0C;

		cTemp = *port[0x0C].lsr;
		cTemp = *port[0x0C].txrx;

		cTemp = *port[cValue].lsr;
		cTemp = *port[cValue].txrx;

		}
	}

and got the following output:

             ; FUNCTION main (BEGIN  RMASK = @0x4022)
                                           ; SOURCE LINE # 33
                                           ; SOURCE LINE # 34
                                           ; SOURCE LINE # 44
0000         ?C0003:
                                           ; SOURCE LINE # 46
                                           ; SOURCE LINE # 48
0000 F2F13000 R    MOV       R1,port+030H
0004 A9A1          MOVB      RL5,[R1]
;---- Variable 'cTemp' assigned to Register 'RL5' ----
                                           ; SOURCE LINE # 49
0006 F2F13200 R    MOV       R1,port+032H
000A A9A1          MOVB      RL5,[R1]
                                           ; SOURCE LINE # 51
000C F2F13000 R    MOV       R1,port+030H
0010 A9A1          MOVB      RL5,[R1]
                                           ; SOURCE LINE # 52
0012 F2F13200 R    MOV       R1,port+032H
0016 A9A1          MOVB      RL5,[R1]
                                           ; SOURCE LINE # 54
0018 0DF3          JMPR      cc_UC,?C0003
             ; FUNCTION main (END    RMASK = @0x4022)

These look AOK to me.

Jon