conversion of Assembly into C : Freescale, DSP56800ERM, assembly language targetting the MC56F83xx microcontroller

May 12, 2008 01:20am pdt

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03.26.2008 at 12:57AM PDT, ID: 23269676 | Points: 500

	[x] Attachment Details

conversion of Assembly into C

Zones: Assembly Programming Language, C Programming Language, Computer CPU Processors

Tags: Freescale, DSP56800ERM, assembly language targetting the MC56F83xx microcontroller

Can someone please help me in figuring out what is going on in the snippet?I'm trying to convert an assembly language code into C.So if I could have the C equivalent of this, that would be great.
thank you

         
             tfr A,B                     /* move A to B */
    neg B                       /* B negated, negative result is converted to positive */
    move.w B1,X0                /* move MSB of B to X0 */
    clr B                       /* clear B */
    move.w X0,B0                /* move X0 to LSB of B  */
    dec.w C                     /* decrement C, C contains ProportionalGainScale -> negative value */   
    asrr.l C,B                  /* shift B to the right by the value contained in C, since the
                                   C is negative B is shifted to the left */

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Question Stats

Zone: Programming

Question Asked By: jurjess

Question Asked On: 03.26.2008

Participating Experts: 2

Points: 500

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03.26.2008 at 01:24AM PDT, ID: 21209690

Rank: Guru

jaime_olivares:

it should be something like this:

short int A, B, C, X; // assumed to be 16 bits

B=A;
B = -B;
X = (X && FF00) | (B >> 8);
B = 0;
B = X & FF;
C--;
B = C>0 ? ( B >> C) : (B << -C);

03.26.2008 at 02:57AM PDT, ID: 21210030

florin_ganea:

I think the snippet is too short to figure out what is trying to do.
It is not easy to make C from it.

A, B and C are 36 bit accumulators (32 bit number, for example A0 [LSP] and A1 [MSP], + 4 bit extension, for example A2), X0 is 16 bit register.

The C code above is not only wrong, but harder to understand than the initial snippet.

Another remark: "dec.w C" is not doing decrement of C, but the decrement of C1, because C is accumulator.

03.26.2008 at 04:39AM PDT, ID: 21210542

jurjess:

A,B and C are 32 bit registers (with 4 bits for sign extension). do you have any idea why this computation would be carried out? I'm trying to look at the reason too.

03.26.2008 at 04:43AM PDT, ID: 21210570

jurjess:

a structure is passes to the function where R2 is the address of the pointer that points to the structure.

         
           
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             sub Y1,Y0            		/* Y0 <= Y0 - Y1 */ 
                              	/*  Y0 <= desiredValue - measuredValue */
 
    move.w X:(R2+0x0006),C 		/* C <= ProportionalGainScale */
 
    tst.l   C                   /* test C (ProportionalGainScale) */
    bge     POSITIVE_PROP_SHIFT  /* if C (ProportionalGainScale) is positive, jump to POSITIVE_PROP_SHIFT. 
                                   Positive C means shift to right, overflow cannot occur */
 
 
    move.w X:(R2),A       		/* A <= ProportionalGain */
    mpy Y0,A1,A          		/* A <= Y0*A1 */
    nop							/* A <= ProportionalGain * (desiredValue - measuredValue) */
 
/* arithmetic left or right shift according to ProportionalGainScale
   value, member of the mc_sPIparams1_limitSc structure */
//---------------------------------------------------------------------
    bge POS_PROP                /* if the result in A is positive, jump to POS_PROP otherwise 
                                   is negative and then continue */
    
    tfr A,B                     /* move A to B */
    neg B                       /* B negated, negative result is converted to positive */
    move.w B1,X0                /* move MSB of B to X0 */
    clr B                       /* clear B */
    move.w X0,B0                /* move X0 to LSB of B  */
    dec.w C                     /* decrement C, C contains ProportionalGainScale -> negative value */   
    asrr.l C,B                  /* shift B to the right by the value contained in C, since the
                                   C is negative B is shifted to the left */
    nop
    tst.w B1                    /* test B1 */
    beq STANDARD_PROP_CALC      /* if B1 is zero, jump to STANDARD_PROP_CALC */
    
    move.l  #0x80000000,A       /* otherwise A is saturated to minimal negative value */
    jmp PROP_PART_SAT           /* jump to PROP_PART_SAT */
    
POS_PROP:                       /* jump here in case of positive A */
    clr B                       /* clear B */
    move.w A1,B0                /* move MSB of A to B0 */
    dec.w C                     /* decrement C, C contains ProportionalGainScale -> negative value */
    asrr.l C,B                  /* shift B to the right by the value contained in C, since the
                                   C is negative B is shifted to the left */
    nop
    tst.w B1                    /* test B1 */
    beq STANDARD_PROP_CALC      /* if B1 is zero, jump to STANDARD_PROP_CALC  */ 
    
    move.l  #0x7FFFFFFF,A       /* otherwise A is saturated to maximal positive value */
    jmp PROP_PART_SAT           /* jump to PROP_PART_SAT */
           
         

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03.26.2008 at 06:32AM PDT, ID: 21211359

florin_ganea:

Just some remarks:
- BGE after MPY checks if there was overflow in 16bit multiplication (MSP of accumulator was set), not if the result is >= 0.
- ASRR.L C, B does not necessarily shifts to the left (only least 5 bits of MSP of C are used)
- DEC.W C may be done for compensation of 2-complement representation of negative numbers (?)

03.26.2008 at 07:03AM PDT, ID: 21211692

jurjess:

ASRR.L shifts to left if the value in register is negative
how do you mean by compensation of negative numers?
also you said the C code representation above by "jaime" was wrong, could you please give me the right version of the C translation to the assembly code I pasted above?

03.26.2008 at 07:06AM PDT, ID: 21211722

florin_ganea:

Yes, you are right about ASSR.L, I was reading wrong documentation.
Also, DEC.W C has nothing to do with negative numbers, as I thought...

03.26.2008 at 07:19AM PDT, ID: 21211852

jurjess:

any help with the code translations?

03.26.2008 at 07:49AM PDT, ID: 21212201

florin_ganea:

Here is the C code, however still not "complete" since the EXT is not taken into account.

         
           
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               short X0 = 0;
  short *A0, *A1, *B0, *B1, *C0, *C1;
  int   A = 0, B = 0, C = 0;
 
  A0 = (short*) &A; A1 = A0 + 1;
  B0 = (short*) &B; B1 = B0 + 1;
  C0 = (short*) &C; C1 = C0 + 1;
 
  *A1 = -1000;
  *C1 = -7;
 
  B = A;
 
  B = -B;
 
  X0 = *B1;
  B = 0;
  *B0 = X0;
  /* equivalent: B = (B >> 16) & 0x0000FFFF; */
 
  (*C1)--;
 
  B = (*C1 > 0) ? (B >> *C1) : (B << -(*C1));

           
         

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03.26.2008 at 07:02PM PDT, ID: 21218005

jurjess:

could you also tell me what the snippet below means?As you can see it is to manage the saturation flag. but it also changes the value of the structure passed to the function this snippet's in.The address of the pointer that points to the base of the structure is in R2 so I dont understand why the filling of register R1 with 0 and R1 with 1?

         
             /* to manage saturation flag, R1=0 & R0 =1 */
    moveu.w #0,R1
    moveu.w #1,R0

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03.27.2008 at 05:49AM PDT, ID: 21220696

florin_ganea:

Well, R0 and R1 belong to AGU and can be used for many things, for example:
- Shadowed for use with fast interrupt processing.
- Modulo arithmetics (R0 and/or R1 are used as source registers in the addressing mode or instruction).
More on this, you can find in the documentation (http://www.freescale.com/files/dsp/doc/ref_manual/DSP56800ERM.pdf)
I cannot realize in which case you are here...

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