frequency table with ascii index and decimal value

I write the code to computer each character in a string's frequency, and the array is index is from 0 to 255, which are ascii index and the value is the frequency that the character appear.

I compare each character in the string and add it 1 in the array each time.

But I cannot find the error that the count seem to be not right(too large)?

The result should be enter image description here

but my result's count is too large.

The error comes in the L1 loop and I don't know why the loop will cause very large number.

Please give me some guidance that what I do wrong.

Thank for your reading.

INCLUDE Irvine32.inc

.data
    testString BYTE "AAEBDCFBBC",0 
    freqTable DWORD 256 DUP(0)
    prompt BYTE 0Dh, 0Ah, 0
    prompt1 BYTE ": ", 0
.code   


Freq PROC uses edi,
        tString:PTR BYTE, 
        fTable:PTR DWORD

    mov eax,0
    CLD

    mov edi,fTable  
    mov ecx,256
    rep stosd;initialize fTable 0


    mov edi,fTable;reset edi position
    mov edx,tString
    mov ecx,SIZEOF tString;element number
    dec ecx;remove null character

    L1:
    mov al,[edx]  ;character value
    inc edx     ;index ++
    inc dword ptr[edi+eax] ;value ++
        Loop L1
    ret
Freq ENDP


main PROC
main ENDP
    INVOKE Freq, ADDR testString, ADDR freqTable
    mov ecx, 256
    mov ebx, 0
    mov edi,OFFSET freqTable
    mov eax, 0

    L1:
        call WriteHex;index
        mov  edx,OFFSET prompt1
        call WriteString;": "

        mov ebx, [edi + eax]
        xchg eax,ebx
        call WriteInt
        xchg eax,ebx
        mov  edx,OFFSET prompt
        call WriteString;endline
        inc eax;index ++
        Loop L1
        ;ret
END main

Solution

From a first glance your code seems about right. I find the line mov ecx,SIZEOF tString a little suspicious, make sure that returns the size of your string - I fear it just returns the size of the pointer. (But that should make the count less not more.)

Also, during the write loop, make sure that the various functions you call don't change any registers you depend on, particularly eax is in danger.

Otherwise you should use a debugger to check whether the produced result is correct so you can tell whether it's the Freq function or your printing loop that's faulty. Having found that, you should step through that part to see where it goes wrong.

Update

Your freqTable is apparently made up of dwords (4 bytes each) but your indexing uses 1-byte units. You'll have to change inc dword ptr[edi+eax] into inc dword ptr[edi+eax*4] in the counting loop, and similarly the mov ebx, [edi + eax] into mov ebx, [edi + eax*4] in the printing loop.