CODEWITHSUNDEEP
assemblyx86tasm
Jamison
Jamison

Reputation: 11

How to iterate over a binary value in x86 assembly using TASM?

I need to display 8 squares on the screen based on a 8-bit binary number. For example, 11111111 would print 8 squares and 10000000 would print just one. I have a subroutine to print each square that works. My problem is I am unsure how to iterate over the binary value to check for ones. If I was using another language like java or C I'd use a for loop to loop over the binary number, checking if each digit was 1, and if it is a one, call the method to print a square. How can I do this in assembly? The value used can come from either memory or a register, but I don't understand how I would go about checking where 1's are in the binary.

Upvotes: 1

Views: 380

Answers (2)

Sep Roland
Sep Roland

Reputation: 39166

Use shl (or shr) to shift out one bit at a time into the carry. If the CF=1 then draw the square.
Next code only loops back if the mask has still at least one ON bit (minimizing the number of iterations):

   mov al, [Mask]

   shl  al, 1
   jnc  .b
.a:
   call DrawSquare
.b:
   shl  al, 1
   jc   .a
   jnz  .b

ps: Make sure DrawSquare does not clobber the AL register. Maybe use a different register.

Upvotes: 3

PaulProgrammer
PaulProgrammer

Reputation: 17630

Try looping 8 times (assuming that you're using a byte value), testing each against an & mask of 0x01 to decide if to print the box, then shifting before going to the next iteration of the loop. Psudeocode below:

for 0..7
  if value & 0x01 == 0x01: print box
  value = value >> 1

Upvotes: 2

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