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Could someone explain?

Ingvar Kreft

This ex. will not work

char	data	test;

char TestF(void);

char TestF(void)
{
if (test == 0x8A)
	{
	//...
	return(1);
	}
}
//Take a look at the Assambly code
             ; FUNCTION TestF (BEGIN)
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
                                           ; SOURCE LINE # 7
                 R     MOV     R7,test
                       MOV     A,R7
                       RLC     A
                       SUBB    A,ACC
                       MOV     R6,A
                       MOV     A,R7
                       XRL     A,#08AH
                       ORL     A,R6
                 R     xJNZ    ?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
                       MOV     R7,#01H
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
             ?C0002:
                       RET
But this one works
char	data	test;

char TestF(void);

char TestF(void)
{
if (test == (char)0x8A)
	{
	//...
	return(1);
	}
}
//Take a look at the Assambly code
             ; FUNCTION TestF (BEGIN)
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
                                           ; SOURCE LINE # 7
                 R     MOV     A,test
                 R     xJNE    A,#08AH,?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
                       MOV     R7,#01H
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
             ?C0002:
                       RET

Parents
  • 0 in reply to Oleg Sergeev

    Ok I can understand that but if I write this

       1          char    data    test;
   2
   3          char TestF(void);
   4
   5          char TestF(void)
   6          {
   7   1      if (test == -118)
   8   1              {
   9   2              //...
  10   2              return(1);
  11   2              }
  12   1      }
  13
  14
CX51 COMPILER V7.09   TEST                                                                 03/04/2004 11:59:16 PAGE 2

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION TestF (BEGIN)
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
                                           ; SOURCE LINE # 7
                 R     MOV     A,test
                 R     xJNE    A,#08AH,?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
                       MOV     R7,#01H
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
             ?C0002:
                       RET
    It will work.
    And we can see that -118 = 0x8a.
    Soo why can't i compare a signed char to an nagative hex value?
    It works from 0x00 - 0x7f.

Reply
  • 0 in reply to Oleg Sergeev

    Ok I can understand that but if I write this

       1          char    data    test;
   2
   3          char TestF(void);
   4
   5          char TestF(void)
   6          {
   7   1      if (test == -118)
   8   1              {
   9   2              //...
  10   2              return(1);
  11   2              }
  12   1      }
  13
  14
CX51 COMPILER V7.09   TEST                                                                 03/04/2004 11:59:16 PAGE 2

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION TestF (BEGIN)
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
                                           ; SOURCE LINE # 7
                 R     MOV     A,test
                 R     xJNE    A,#08AH,?C0002
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
                       MOV     R7,#01H
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
             ?C0002:
                       RET
    It will work.
    And we can see that -118 = 0x8a.
    Soo why can't i compare a signed char to an nagative hex value?
    It works from 0x00 - 0x7f.

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