Can You Compare Memory Operands in Assembly?

Understanding how data is manipulated and compared within memory is crucial for assembly programming. This article delves into comparing memory operands in assembly language, illustrating how to sort byte-sized elements using an in-register bubble sort algorithm. We’ll examine the process of loading data into registers, performing comparisons, and storing the sorted results back into memory.

Loading Data into Registers for Comparison

To compare memory operands, we first need to load them into registers. For efficiency, multiple bytes can be loaded simultaneously using word-sized registers when dealing with smaller data sets. Let’s consider sorting seven bytes:

push    ebx          ; Preserve the value of EBX
mov     ax, word ptr MyArray[0] ; Load the first two bytes into AX
mov     cx, word ptr MyArray[2] ; Load the next two bytes into CX
mov     dx, word ptr MyArray[4] ; Load the next two bytes into DX
mov     bl, MyArray[6]  ; Load the last byte into BL

This code snippet utilizes word ptr to override the byte definition of the array, allowing us to load two bytes at a time into the 16-bit registers AX, CX, and DX. The remaining byte is loaded into the 8-bit register BL. We preserve the EBX register because we’ll be using its lower byte (BL). Registers EAX, ECX, and EDX are generally considered volatile and their prior content is not expected to be preserved.

Implementing the In-Register Bubble Sort

With the data loaded, we can implement a bubble sort algorithm directly within the registers. This involves pairwise comparisons and swaps to progressively order the elements. While a loop would be used for larger arrays, for this example, we’ll unroll the loop to demonstrate the core logic:

; First Pass (Bottom to Top) - Largest element ends up in BL
.if al > ah      xchg al, ah .endif
.if ah > cl      xchg ah, cl .endif
.if cl > ch      xchg cl, ch .endif
.if ch > dl      xchg ch, dl .endif
.if dl > dh      xchg dl, dh .endif
.if dh > bl      xchg dh, bl .endif

; Second Pass (Top to Bottom) - Smallest element ends up in AL
.if dh < dl      xchg dh, dl .endif  
.if dl < ch      xchg dl, ch .endif
.if ch < cl      xchg ch, cl .endif
.if cl < ah      xchg cl, ah .endif
.if ah < al      xchg ah, al .endif

; Subsequent passes continue until the array is sorted...

Each pass moves the largest unsorted element towards the top (BL) or the smallest element to the bottom (AL). The process repeats with progressively fewer comparisons until all elements are sorted.

Storing the Sorted Data Back into Memory

Once the in-register sorting is complete, the ordered bytes are stored back into the original memory location:

mov word ptr MyArray[0], ax  ; Store the first two sorted bytes
mov word ptr MyArray[2], cx  ; Store the next two sorted bytes
mov word ptr MyArray[4], dx  ; Store the next two sorted bytes
mov MyArray[6], bl           ; Store the last sorted byte
pop ebx                      ; Restore the original value of EBX

This completes the sorting process, demonstrating how memory operands can be compared and manipulated within assembly language. This example illustrates a fundamental concept applicable to more complex sorting and data manipulation tasks. By understanding these basic principles, developers can write highly optimized and efficient code for low-level operations.