CODEWITHSUNDEEP
variablesassemblydeclarationx86-16emu8086
tagurit
tagurit

Reputation: 558

Error: "unknown opcode skipped: 32"

I wrote an 8086 program, and as far as I can tell it runs fine, but when it gets to the part where I declare the variables, the emulator gives me an error. When trying to run the line temp db 0x0F, the emulator says:

unknown opcode skipped: 32

not 8086 instruction - not supported yet.

Here's my full program:

org 100h
mov ah, temp ;put variables into registers
mov al, changed
mov dx, result
lea bx, temp ;get address of temp and put into bx
add dx, [bx] ;add value at the address in bx to result
lea bx, changed ;get address of changed and put into bx
add dx, [bx] ;add value at the address in bx to result
temp db 0x0F ;declare and initialize variables
changed db 32h
result dw 0

Is this consequential to how the program functions, and how do I fix it?

EDIT: sigjuice solved the problem, as you can see in the comments. Here's the final version of the program that runs correctly:

.CODE
org 100h
mov ah, temp ;put variables into registers
mov al, changed
mov dx, result
lea bx, temp ;get address of temp and put into bx
add dx, [bx] ;add value at the address in bx to result
lea bx, changed ;get address of changed and put into bx
add dx, [bx] ;add value at the address in bx to result
.DATA
temp db 0x0F ;declare and initialize variables
changed db 32h
result dw 0

Upvotes: 0

Views: 895

Answers (1)

Sep Roland
Sep Roland

Reputation: 39166

add dx, [bx] ;add value at the address in bx to result
temp db 0x0F ;declare and initialize variables

In this part of your program there's nothing that stops the CPU from executing the data at the temp label as if it were an instruction.

Although adding the .CODE and .DATA assembler directives (perhaps suggested by @sigjuice) seemingly solves the problem, this is typically not what you use when writing a .COM executable. It's a .COM executable because you used the org 100h directive.

What your program really needs is a way to return to the operating system. Since this is EMU8086 the preferred way is using the DOS.TerminateWithReturncode function.

add  dx, [bx]   ;add value at the address in bx to result
                ; Exit to the operating system
mov  ax, 4C00h  ;AH=4Ch function number, AL=0 exitcode (0 most often means OK)
int  21h        ;DOS system call
                ; Now beyond this point nothing gets executed inadvertently
temp db 0Fh     ;declare and initialize variables

I can't really advice to return to the operating system using a mere ret instruction, because this method requires that the SS:SP registers are set as they were when the program started. This will not always be the case. Better use this DOS function that does not rely on any specific register setting.

lea bx, temp ;get address of temp and put into bx
add dx, [bx] ;add value at the address in bx to result
lea bx, changed ;get address of changed and put into bx
add dx, [bx] ;add value at the address in bx to result

Nothing to do with your original problem but as a bonus:

Because temp and changed are both byte-sized variables, the word-sized additions don't just add the variables alone but also the byte that happens to follow them in memory! Sometimes this is intentional (I sincerily doubt this is the case here!), but you need to make sure that you understand this.

Upvotes: 1

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