使用通过参数传递的偏移量计算程序内部数组的长度，汇编语言 x8086

Question

So i am trying to get the length of array by using the offset in the parameter but it is only returning lenthof offset. Is there any way I can do this? `

INCLUDE Irvine32.inc
multiply proto,arr:ptr dword
.data
array dword 1,2,3,4,5,6,7,8,9,10
num dword 3
.code
main PROC
    invoke multiply,addr array
    exit
main ENDP

multiply proc,arr:ptr dword
    mov ecx,lengthof arr
    mov eax, ecx
    call writedec


    ret
multiply endp

END main

`

Answer 1

Most assemblers allow you to do compile-time "label arithmetic" (as I like to call it) where you can subtract one label from another to get the distance between them measured in bytes. The syntax may vary depending on the assembler but usually it will look something like this:

array dword 1,2,3,4,5,6,7,8,9,10
arrayEnd label byte  ;this exists just to mark the end


mov ecx, offset (arrayEnd-array)  ;should equal 40 since each entry is 4 bytes.
mov eax,ecx
call writedec

If you use this technique, the distance is always measured in bytes. Unlike C, assembly does not auto-scale pointer arithmetic by the size of the data type you're working with. Keep this in mind when indexing the array, or you'll be loading the wrong values.

There are limitations to this method, however. For one it doesn't work with malloc or any runtime memory allocation like that. This method is purely a compile-time constant, it's the equivalent of just counting them yourself (in your head) and writing that constant in your code as a "magic number."

Also, it can't tell the difference between "empty slots" in your array. For example:

array dword 1,2,3,4,5,0,0,0,0,0
arrayEnd label byte

Let's say you intended the last five zeroes as "free space" for more data, and when you measure the size you only want to count data that isn't "free space." The arrayEnd-array method won't help you with that.