Assembly Language Program Analysis: Finding Register Values

This problem asks us to determine the final values of the ax and cx registers after executing a given assembly language program fragment. Let's break down the program line by line and trace the execution.

Understanding the Assembly Code Fragment

The program fragment is as follows:

mov ax, 0h
mov cx, 0A h
doloop:
dec ax
loop doloop

Here's what each instruction does:

• mov ax, 0h: This instruction moves the hexadecimal value 0h into the ax register. So, initially, ax is set to 0.
• mov cx, 0A h: This instruction moves the hexadecimal value 0A h into the cx register. 0A h is equivalent to decimal 10. The cx register is often used as a counter for loops.
• doloop:: This is a label that marks the beginning of the loop.
• dec ax: This instruction decrements the value in the ax register by 1.
• loop doloop: This instruction performs two actions:
  1. It decrements the cx register by 1.
  2. It checks if the value in cx is non-zero. If cx is not zero, it jumps back to the label specified (doloop). If cx is zero, execution continues with the next instruction after the loop instruction.

Tracing the Program Execution

Let's trace the values of ax and cx through each iteration of the loop.

Initial state:

ax = 0h

cx = 0A h (which is 10 in decimal)

The loop doloop instruction will cause the code between the doloop label and the loop instruction (which is just dec ax) to execute cx times. Since cx starts at 10, the loop body will execute 10 times.

Let's see how the register values change during the loop iterations:

Alternatively, since ax starts at 0 and is decremented 10 times, the final value can be calculated as $0 - 10$. In 16-bit two's complement representation, $0 - 10$ is equal to FFF6h.

The loop terminates when cx becomes 0. The loop instruction decrements cx from 1 down to 0, and then the loop finishes.

Final Register Values

After the loop completes, the value of ax is FFF6h, and the value of cx is 0h.

Conclusion

By tracing the execution of the assembly code fragment, we found that the ax register is decremented 10 times, starting from 0, resulting in FFF6h. The cx register is used as a loop counter and is decremented from 0A h (10) down to 0, at which point the loop terminates.

Revision Table: Assembly Language Basics

Additional Information: Assembly Loop Concepts

• The loop instruction is a composite instruction that combines decrementing cx and a conditional jump.
• The loop instruction is efficient for simple counted loops where the count is in cx.
• Other conditional loop instructions exist, such as loope/loopz (loop while equal/zero flag is set) and loopne/loopnz (loop while not equal/not zero flag is set), which also decrement cx but additionally check a flag condition.
• Register sizes: In 16-bit assembly (like for the 8086 processor), AX and CX are 16-bit registers. They can hold values from 0 to FFFFh (65535 decimal) for unsigned numbers, or -32768 to +32767 for signed numbers using two's complement.