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shift with ">>" in c51 ...

some time c user do shift for mul and divide and some time u write byte to byte
then not require any carry for me in asm i do two shift mul 4

i do it with c51

ypos3 = ypos>>3;
; SOURCE LINE # 237
MOV A,R5
RRC A
RRC A
ANL A,#01FH
;---- Variable 'ypos3?1255' assigned to

HOW DO IT IN C51 WITHOUT
MOV A,R5
ANL A,#01FH

??

Parents

0 Graham Cole over 23 years ago in reply to Mathias Neuhaus

For this C code:

unsigned char t;
...

    t = t << 3;

    t = _crol_(t,3);

Keil C51 (6.14) will give you this:

                                           ; SOURCE LINE # 988
                                           ; SOURCE LINE # 991
0005 900000      R     MOV     DPTR,#t
0008 E0                MOVX    A,@DPTR
0009 FF                MOV     R7,A
000A 33                RLC     A
000B 33                RLC     A
000C 33                RLC     A
000D 54F8              ANL     A,#0F8H
000F FF                MOV     R7,A
0010 F0                MOVX    @DPTR,A
                                           ; SOURCE LINE # 993
0011 7803              MOV     R0,#03H
0013 08                INC     R0
0014 8001              SJMP    ?C0090
0016         ?C0089:
0016 23                RL      A
0017         ?C0090:
0017 D8FD              DJNZ    R0,?C0089
0019 900000      R     MOV     DPTR,#t
001C F0                MOVX    @DPTR,A

But Mathias is quite right to point out that a single byte in the accumulator can be rotated by any number of bits with no more than TWO instructions.

To rotate n bits to the left:

n%8
0    NOP
1    RL A
2    RL A,     RL A
3    SWAP Acc, RR A
4    SWAP Acc
5    SWAP Acc, RL A
6    RR A,     RR A
7    RR A

A final masking operation will convert a rotate into a shift operation.

Keil users might be under the impression that the intrinsic functions are fast and/or compact - they are a little better than a portable C-based algorithm, but they not optimal. Shifting or rolling by a constant number of bits are common operations, sometimes used to achieve fast multiplication or division by 2^n. Shifts and rolls by exactly one bit are common operations required for CRCs, LFSR etc. It really would be nice if the C51 optimiser could do a better job in this area.

Reply

0 Graham Cole over 23 years ago in reply to Mathias Neuhaus

For this C code:

unsigned char t;
...

    t = t << 3;

    t = _crol_(t,3);

Keil C51 (6.14) will give you this:

                                           ; SOURCE LINE # 988
                                           ; SOURCE LINE # 991
0005 900000      R     MOV     DPTR,#t
0008 E0                MOVX    A,@DPTR
0009 FF                MOV     R7,A
000A 33                RLC     A
000B 33                RLC     A
000C 33                RLC     A
000D 54F8              ANL     A,#0F8H
000F FF                MOV     R7,A
0010 F0                MOVX    @DPTR,A
                                           ; SOURCE LINE # 993
0011 7803              MOV     R0,#03H
0013 08                INC     R0
0014 8001              SJMP    ?C0090
0016         ?C0089:
0016 23                RL      A
0017         ?C0090:
0017 D8FD              DJNZ    R0,?C0089
0019 900000      R     MOV     DPTR,#t
001C F0                MOVX    @DPTR,A

But Mathias is quite right to point out that a single byte in the accumulator can be rotated by any number of bits with no more than TWO instructions.

To rotate n bits to the left:

n%8
0    NOP
1    RL A
2    RL A,     RL A
3    SWAP Acc, RR A
4    SWAP Acc
5    SWAP Acc, RL A
6    RR A,     RR A
7    RR A

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