error A2022: instruction operands must be the same size

Question

Hey so I am getting this error when I run this code:

1>------ Build started: Project: Project, Configuration: Debug Win32 ------
1>  Assembling [Inputs]...

1>assign2.asm(32): error A2022: instruction operands must be the same size

1>assign2.asm(33): error A2022: instruction operands must be the same size

1>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\BuildCustomizations\masm.targets(49,5):

 error MSB3721: The command "ml.exe /c /nologo /Zi /Fo"Debug\assign2.obj" /Fl"Project.lst" 

/I "c:\Irvine" /W3 /errorReport:prompt  /Taassign2.asm" exited with code 1.

========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Here is the following code, it happens when I try to subtract ml1337skillz from usPop and store that result into Difference. (Keep in mind I am just starting to learn this area :))

TITLE Learning      (learning.asm)

INCLUDE Irvine32.inc

secondE = 00000000010001010000000000000000b

.data

myName BYTE "John Smith"

nameLen = ($ - myName)

nationalDebt QWORD 7975482431119.47d

usPop DWORD 313900000d

kindaNegative SDWORD -2147483648d

my1337Sk1LLz WORD 1337h

negUnit SBYTE -1

half BYTE 0.5d

Difference SWORD ?

starField BYTE 2000 DUP("*")

bigLoss SDWORD -50000

.code
main PROC

FillRegs:

    mov eax,usPop           ;store 3139000000d into eax
    sub eax,my1337Sk1LLz    ;subtracts 1337h from usPop in eax
    mov Difference, eax     ;stores eax into Difference

    mov edx,bigLoss         ;stores -50000 into edx

    mov eax,secondE         ;store 0E00 into eax
    mov ah,'A'              ;store string A into ah 
    mov al,'X'              ;store string X into al

    mov ebx,secondE         ;store 0E00 into ebx
    mov bh,'B'              ;store string B into bh
    mov bl,'X'              ;store string X into bl

    call DumpRegs           ;shows Registers

    exit                    ;exits
    main ENDP

END main

Answer 1

These two lines are your problem:

sub eax,my1337Sk1LLz    ;subtracts 1337h from usPop in eax
mov Difference, eax     ;stores eax into Difference

eax is 32 bits, but both my1337Sk1LLz and Difference are 16 bits.

There are two ways you might get around this:

1. Changing the size of my1337Sk1LLz and Difference. Right now you have the types as WORD and SWORD, respectively. You can change those to DWORD and SDWORD to make them 32-bit.

2. Zero-extending and truncating. You'll need another register. I'll use edx since you don't seem to be using it there. First, you'll need to sign-extend my1337Sk1LLz:

   movzx edx, my1337Sk1LLz  ; move, zero-extended, my1337Sk1LLz into EDX
   

   Then you can do the subtraction:

   sub eax, edx  ; they're the same size now so we can do this
   

   Then you can store the low word of eax into Difference, discarding the high word:

   mov Difference, ax