assembly convert number into character

I have this variable:

Message DB 10 dup(?)

I'm trying to generate 10 random characters and then save them into variable.

mov ecx,10
mov edi,0

GenerateString:
    mov eax,60h
    call RandomRange
    sub eax,27
    add al,'0'
    mov Message[edi],al

    inc edi
    loop GenerateString

mov edx,offset Message
call WriteString

I found somewhere that I have to substract number by 27 and add ascii 0, but it's not working properly. Please note I have to use RandomRange.

Solution

Irvine's RandomRange creates numbers in the range 0..EAX-1. With other words: You get a number within a range of a certain amount of different numbers. Since you want an ASCII character in the range a..z (97..122) you have

to determine the size of the range: 122-97+1 = 26 characters,
to move the range of RandomRange to the desired range by adding the start value: 97.

BTW: You will always get the same sequence if you don't initialize RandomRange with Randomize.

INCLUDE Irvine32.inc

.DATA
    Message DB 10 dup(0)

.CODE
main PROC

    call Randomize                      ; Initialization for `RandomRange`

    mov ecx, LENGTHOF Message - 1       ; Without the terminating null!
    mov edi, 0

    GenerateString:

    mov eax, 26                         ; Range: [0..25] = 26 numbers
    call RandomRange
    add eax, 97                         ; Move the range from [0..25] to [97..122]
    mov Message[edi],al

    inc edi
    loop GenerateString

    mov edx,offset Message
    call WriteString

    exit

main ENDP

END main