I am taking assembly language class and have a project but I'm somewhat stuck and can't find what is wrong in my code. The project asks me to write a couple basic arithmetic operation (addition and subtract) based on the 2 values that are divided from my student ID.
My student ID has 7 digits and the 3 most significant digits are stored in variable "left" that is moved to register EAX, the 4 least significant digits are stored in variable "right" that is moved to register EBX. I'm asked to find the total and positive difference from the 2 values above and then store the total value and positive difference value to 2 different registers (ECX and EDX respectively).
I already found the total using ADD instruction just find. However, when I do the positive difference using SUB instruction, all I get is the "left" value or the negative difference. Below is the code:
mov eax,left ;// get first 3 digits
call DumpRegs ;// DumpReg to display the contents of the register
mov ebx,right ;// get last 4 digits
call DumpRegs
add eax, ebx ;// add both values together
call DumpRegs
mov total,eax
call DumpRegs
mov ecx,total
call DumpRegs
mov eax,left
call DumpRegs
call WriteInt
mov eax,left
call DumpRegs
call WriteInt
mov ebx,right
call DumpRegs
sub ebx,eax
mov diff,ebx
mov edx,diff
mov ebx,right
call DumpRegs
For example, if my student ID is 1234567, then left is 123 and 4567 is right. So eax will be 123 and ebx will be 4567. The total of eax and ebx will be a number stored to "total" which is registered to ecx. Then the positive difference I got according to my code is 123. If I switch
sub ebx,eax
to
sub eax,ebx
I then get a negative value. I can't figure out why the sub only show the left value.
Edit: this is the content of the registers after each instruction
EAX=000000F2 EBX=7F0EF000 ECX=012A1055 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A352A EFL=00000246 CF=0 SF=0 ZF=1 OF=0 AF=0 PF=1
EAX=000000F2 EBX=00001860 ECX=012A1055 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A3535 EFL=00000246 CF=0 SF=0 ZF=1 OF=0 AF=0 PF=1
EAX=00001952 EBX=00001860 ECX=012A1055 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A353C EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
EAX=00001952 EBX=00001860 ECX=012A1055 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A3546 EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
+6482
EAX=00001952 EBX=00001860 ECX=00001952 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A3556 EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
EAX=000000F2 EBX=00001860 ECX=00001952 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A3560 EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
+242
EAX=000000F2 EBX=00001860 ECX=00001952 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A356F EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
+242
EAX=000000F2 EBX=00001860 ECX=00001952 EDX=012A1055
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A357F EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
+242
EAX=000000F2 EBX=00001860 ECX=00001952 EDX=0000176E
ESI=012A1055 EDI=012A1055 EBP=006BFC54 ESP=006BFC44
EIP=012A359D EFL=00000202 CF=0 SF=0 ZF=0 OF=0 AF=0 PF=0
Hello world!☺Press any key to continue . . .
Running this code:
mov left, 123
mov right, 4567
mov eax,left
call DumpRegs
call WriteInt
mov ebx,right
call DumpRegs
sub ebx,eax
mov diff,ebx
Correctly produces the positive difference 4444. If it doesn't for you, please edit your question so it includes the output of all those DumpRegs calls.