Alp in MASM 8086

Question

If the number is present, the output should show the numbers that are not present.

.model small
.Stack 100
.data
 array db 5, 12, 23, 45, 60, 67, 96, 100
 len equ $-array
 key db 100
 msg1 db "found$"
 msg2 db "not found$"
.code
 mov ax, @data 
 mov ds, ax 
 mov si, 00h 
 mov di, len-1 
 mov cl, key 
again: mov bx, si 
 add bx, di 
 shr bx, 1 
 cmp cl, array[bx]
 je found 
 ja down 
 mov di, bx 
 dec di 
 jmp up 
down: mov si, bx 
 inc si 
up: cmp si, di 
 jle again 
 lea dx, msg2 
 jmp display 
found: lea dx, msg1 
display: mov ah, 09h 
 int 21h 
 int 3 
 end 

Answer 1

• It's important to provide comments in assembly, especially for register usage. Since registers cannot have unique names like variables, readers are left to deduce their purpose.

  You may, for instance, state the value set to a register in a certain line, and what that register is supposed to represent at that time. That way, you can keep track of the register's usage as it changes throughout the program.

• Your variable initializations should have equal whitespace on both sides of the variable size:

  array db 5, 12, 23, 45, 60, 67, 96, 100
  

• This part is misaligned:

  .code
   mov ax, @data 
   mov ds, ax
  

  it should be:

  .code
   mov ax, @data 
   mov ds, ax
  

  Same thing here:

  found: lea dx, msg1 
  display: mov ah, 09h 
   int 21h 
   int 3 
   end
  

  it should be:

  found: lea dx, msg1 
  display: mov ah, 09h 
   int 21h 
   int 3 
  end
  

  I'd also add a linebreak between those two procedures.

Answer 2

.model small

Hard to imagine somebody writing new code for DOS, but okay...

 msg1 db "found$"
 msg2 db "not found$"

Seems to me these deserve better names:

fnd_msg db "found$"
not_fnd_msg db "not found$"

Or, you can get kind of tricky to save a little space:

not_fnd_msg db "not "
fnd_msg db "found$"

I'm somewhat hesitant to suggest overlapping the storage for the two strings this way. Under almost any other circumstances I'd probably frown on it, but if you're going to write for MS-DOS, using otherwise ugly tricks to save a little space is often nearly a necessity.

 mov si, 00h 

You typically want to use xor or sub to clear a register:

xor si, si

In this case, however, you can probably skip that entirely -- you don't really need to use si at all. Especially in 16-bit mode, you typically get some real benefit from using the registers as they were designed, such as a count in cx.

 mov di, len-1 
 mov cl, key 

As such, I'd probably use:

mov cx, len-1
mov al, key
mov di, array

This improves readability considerably (at least for others who know what they're doing). If you're going to put the value to compare in cl, the count in di, and so on, you just about need to add a comment to point out that you're doing something unusual (and preferably, why). If you use the registers as intended, there's no real need for those comments, because the values are exactly where anybody reading the code will expect them to be. If you explicitly stated that you'd stored the count in cx, there's a pretty fair chance somebody reading it would think you were insulting their intelligence by pointing out the painfully obvious.

Then you can do the search with:

repne scasb

Once that's finished, you can use the z flag to see if the data was found or not. To avoid ugly jumps on both legs of an if/then/else type of construction, you can use a little trick. It depends on the fact that on an x86, a mov does't affect the flags. As such, you can do a mov of a default value, then do a conditional branch, and afterwards load the other possible value:

 mov bx, offset fnd_msg
 jz prnt_msg
 mov bx, offset not_fnd_msg
prnt_msg:
 mov ah, 09h
 int 21h

Although the logic here may not be immediately apparent to "outsiders", almost anybody accustomed to assembly language will recognize this very quickly (and if you don't use it, they'll wonder why, or just assume you don't know what you're doing).

Then you probably want to exit normally, not via a debug interrupt:

mov ax, 4c00h
int 21h

• Ruslan: Good answer concerning code itself, not just formatting. Alternatively to using 4C DOS service, one could just do int 20h for normal termination.