This is the code generated by the RealView compiler for a re-written loop. It takes 43.57us at 58.9MHz

   165:         unsigned char *pport, *pbuf; 
   166:         unsigned int i; 
   167:         for (i = 256, pport = &FIO_PIN[3], pbuf = buf; i--; ) 
0x00000610  E3A09C01  MOV       R9,#0x00000100
0x00000614  E59F73BC  LDR       R7,[PC,#0x03BC]
0x00000618  E28D8000  ADD       R8,R13,#0x00000000
0x0000061C  EA000001  B         0x00000628
   168:             *pport = *pbuf++; 
   169:  
0x00000620  E4D80001  LDRB      R0,[R8],#0x0001
0x00000624  E5C70000  STRB      R0,[R7]
0x00000628  E1B00009  MOVS      R0,R9
0x0000062C  E2499001  SUB       R9,R9,#0x00000001
0x00000630  1AFFFFFA  BNE       0x00000620

 

This is the output from the Keil compiler. It takes 61.02us so does not satisfy Richard's criterion.

   165:         unsigned char *pport, *pbuf; 
   166:         unsigned int i; 
   167:         for (i = 256, pport = &FIO_PIN[3], pbuf = buf; i--; ) 
   168:             *pport = *pbuf++; 
0x000003F0  4BB1      LDR       R3,[PC,#0x02C4]
0x000003F2  4AB2      LDR       R2,[PC,#0x02C8]
0x000003F4  A900      ADD       R1,SP,#0x0000
0x000003F6  1C08      ADD       R0,R1,#0
0x000003F8  E004      B         0x00000404
0x000003FA  1C05      ADD       R5,R0,#0
0x000003FC  3001      ADD       R0,#0x01
0x000003FE  782E      LDRB      R6,[R5,#0x00]
0x00000400  1C15      ADD       R5,R2,#0
0x00000402  702E      STRB      R6,[R5,#0x00]
0x00000404  1C1D      ADD       R5,R3,#0
0x00000406  3B01      SUB       R3,#0x01
0x00000408  2D00      CMP       R5,#0x00
0x0000040A  D1F6      BNE       0x000003FA

 

As a comparison this code takes 5641 cycles with the C51 compiler (169us with Richards super 8051). So I can quite see why Richard is so disparaging about 8051 C compilers.

    unsigned char xdata buf[256], *pbuf; 
    unsigned int i; 
    for (i = 0; i < 256; ) 
        P1 = buf[i++]; 

 

Using pointers is incredibly slow. Which reinforces the comments about bit manipulations being good on an 8051, and strings, arithmetic etc better on other mcus.

David.