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assembler rtn called from C

I have the impression that an assembler non-interrupt subroutine called from 'C' need save no registers.

Is that correct?

Erik

Parents

0 Jon Young over 23 years ago in reply to erik malund

The point is the REGUSE control in the assembly code makes a big difference in the C code that calls the assembly code!!

In the following example the calling C code was reduced by almost 1/2 in size by using REGUSE on the assembly side.

Main.c

void WithoutRegUse( char C );
void WithRegUse( char C );

void CallWithoutRegUse( void )
{
  char i;
  char xdata* p = 0;
  for( i = 0; i != 0x12; ++i, ++p )
    WithoutRegUse(*p);
}

void CallWithRegUse( void )
{
  char i;
  char xdata* p = 0;
  for( i = 0; i != 0x12; ++i, ++p )
    WithRegUse(*p);
}

void main( void )
{
  CallWithoutRegUse();
  CallWithRegUse();
}

Sub.c

#include <reg52.h>

volatile V;

void WithoutRegUse( char C )
{
  ACC = C;
  #pragma asm
    swap a
    mov  V,a
  #pragma endasm
  return;
}

#pragma asm
  $reguse _WithRegUse( A, R7 ) 
#pragma endasm
void WithRegUse( char C )
{
  ACC = C;
  #pragma asm
    swap a
    mov  V,a
  #pragma endasm
  return;
}

Partial Main.lst

             ; FUNCTION CallWithoutRegUse (BEGIN)
                                           ; SOURCE LINE # 7
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
0000 E4                CLR     A
0001 F500        R     MOV     p,A
0003 F500        R     MOV     p+01H,A
                                           ; SOURCE LINE # 11
0005 F500        R     MOV     i,A
0007         ?C0001:
0007 E500        R     MOV     A,i
0009 6412              XRL     A,#012H
000B 6017              JZ      ?C0004
                                           ; SOURCE LINE # 12
000D 850082      R     MOV     DPL,p+01H
0010 850083      R     MOV     DPH,p
0013 E0                MOVX    A,@DPTR
0014 FF                MOV     R7,A
0015 120000      E     LCALL   _WithoutRegUse
0018 0500        R     INC     i
001A 0500        R     INC     p+01H
001C E500        R     MOV     A,p+01H
001E 70E7              JNZ     ?C0001
0020 0500        R     INC     p
0022         ?C0010:
0022 80E3              SJMP    ?C0001
                                           ; SOURCE LINE # 13
0024         ?C0004:
0024 22                RET     
             ; FUNCTION CallWithoutRegUse (END)

             ; FUNCTION CallWithRegUse (BEGIN)
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
                                           ; SOURCE LINE # 18
;---- Variable 'p' assigned to Register 'DPTR' ----
0000 900000            MOV     DPTR,#00H
                                           ; SOURCE LINE # 19
;---- Variable 'i' assigned to Register 'R6' ----
0003 E4                CLR     A
0004 FE                MOV     R6,A
0005         ?C0005:
0005 EE                MOV     A,R6
0006 6412              XRL     A,#012H
0008 6009              JZ      ?C0008
                                           ; SOURCE LINE # 20
000A E0                MOVX    A,@DPTR
000B FF                MOV     R7,A
000C 120000      E     LCALL   _WithRegUse
000F 0E                INC     R6
0010 A3                INC     DPTR
0011 80F2              SJMP    ?C0005
                                           ; SOURCE LINE # 21
0013         ?C0008:
0013 22                RET     
             ; FUNCTION CallWithRegUse (END)

Reply

0 Jon Young over 23 years ago in reply to erik malund

Main.c

void WithoutRegUse( char C );
void WithRegUse( char C );

void CallWithoutRegUse( void )
{
  char i;
  char xdata* p = 0;
  for( i = 0; i != 0x12; ++i, ++p )
    WithoutRegUse(*p);
}

void CallWithRegUse( void )
{
  char i;
  char xdata* p = 0;
  for( i = 0; i != 0x12; ++i, ++p )
    WithRegUse(*p);
}

void main( void )
{
  CallWithoutRegUse();
  CallWithRegUse();
}

Sub.c

#include <reg52.h>

volatile V;

void WithoutRegUse( char C )
{
  ACC = C;
  #pragma asm
    swap a
    mov  V,a
  #pragma endasm
  return;
}

#pragma asm
  $reguse _WithRegUse( A, R7 ) 
#pragma endasm
void WithRegUse( char C )
{
  ACC = C;
  #pragma asm
    swap a
    mov  V,a
  #pragma endasm
  return;
}

Partial Main.lst

             ; FUNCTION CallWithoutRegUse (BEGIN)
                                           ; SOURCE LINE # 7
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 10
0000 E4                CLR     A
0001 F500        R     MOV     p,A
0003 F500        R     MOV     p+01H,A
                                           ; SOURCE LINE # 11
0005 F500        R     MOV     i,A
0007         ?C0001:
0007 E500        R     MOV     A,i
0009 6412              XRL     A,#012H
000B 6017              JZ      ?C0004
                                           ; SOURCE LINE # 12
000D 850082      R     MOV     DPL,p+01H
0010 850083      R     MOV     DPH,p
0013 E0                MOVX    A,@DPTR
0014 FF                MOV     R7,A
0015 120000      E     LCALL   _WithoutRegUse
0018 0500        R     INC     i
001A 0500        R     INC     p+01H
001C E500        R     MOV     A,p+01H
001E 70E7              JNZ     ?C0001
0020 0500        R     INC     p
0022         ?C0010:
0022 80E3              SJMP    ?C0001
                                           ; SOURCE LINE # 13
0024         ?C0004:
0024 22                RET     
             ; FUNCTION CallWithoutRegUse (END)

             ; FUNCTION CallWithRegUse (BEGIN)
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 16
                                           ; SOURCE LINE # 18
;---- Variable 'p' assigned to Register 'DPTR' ----
0000 900000            MOV     DPTR,#00H
                                           ; SOURCE LINE # 19
;---- Variable 'i' assigned to Register 'R6' ----
0003 E4                CLR     A
0004 FE                MOV     R6,A
0005         ?C0005:
0005 EE                MOV     A,R6
0006 6412              XRL     A,#012H
0008 6009              JZ      ?C0008
                                           ; SOURCE LINE # 20
000A E0                MOVX    A,@DPTR
000B FF                MOV     R7,A
000C 120000      E     LCALL   _WithRegUse
000F 0E                INC     R6
0010 A3                INC     DPTR
0011 80F2              SJMP    ?C0005
                                           ; SOURCE LINE # 21
0013         ?C0008:
0013 22                RET     
             ; FUNCTION CallWithRegUse (END)

Children

0 Graham Cole over 23 years ago in reply to Jon Young

Too right. Thanks for the nice examples.
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