# Key Points on Programming in Assembler

### Prerequisite

This article assumes that the reader has installed MASM32. If they have not, it is available from http://www.masm32.com/.

### Introduction

In this third and final part of this tutorial, I will cover the common arithmetic functions and some of the macros in MASM that considerably aid in the implementation of Assembler. Go to An Introduction to Assembly Language Programming if you would like to start at the beginning of the series.

### Increment and Decrement

**inc**and

**dec**. For example:

TestProc proc mov eax, 5 dec eax dec eax mov dl, 10 inc dl inc dl ret TestProc endp

When using decrement **dec**, if the result is zero, the zero flag is set. This can be used to implement loops. For example:

TestProc proc mov ecx, 10 xor eax, eax ; efficient way of saying eax=0 LoopStart: inc eax dec ecx jnz LoopStart ; eax now equals 10 ret TestProc endp

### Addition and Subtraction

The addition and subtraction instructions are **add** and **sub**. They take the general form of:

add/sub (destination), (source)

You can add registers to registers, registers to constants, and registers to the contents of memory. The size (in bits) of the source and destinations have to be the same. Both affect the flags of the system. For instance, if the result of a **sub** is zero, the zero flag is set. For example:

AddValues proc dwValue1:DWORD, dwValue2:DWORD mov eax, dwValue1 add eax, dwValue2 ret AddValues endp

This method adds the two values passed in and returns the result.

### Multiplication and Division

The instructions for multiplication and division are **mul** and **div**. Both only operate on the accumulator register (eax) and use the data register (edx) as an overflow. The part of the registers affected are determined by the size of the operand.

The following diagram demonstrates how the accumulator and the data registers fit together when being used by the instructions.

Therefore, to get expected results, it is recommended that you set edx to zero before calling **mul** or **div**. For example:

TestProc proc mov eax, 10 xor edx, edx ; set edx to zero mul 10 div 10 ret TestProc endp

### Logical Operations

The usual logical operations are convered by **or**, **and**, and **xor**. They take the form as follows:

logical operation (destination), (source)

The size in bits of the source and and destination have to be the same. For instance:

LogicalFunction proc xor eax, eax ; the efficient way of saying eax=0 mov ax, 100 mov bx, 5 and ax, 1 or ax, bx ret LogicalFunction endp

### Bitshift Operations

The instructions **shl** and **shr** shift the given register bits left and right by the given bit count. These are highly efficient, and should be preferred over the **mul** and **div** instructions for parameters that are powers of two. For example:

ShiftFunction proc mov eax, 1 shl eax, 2 ; shift eax's bits left 2 times : i.e. eax *= 4 shr eax, 2 ; shift eax's bits right 2 times : i.e. eax /= 4 ret ShiftFunction endp

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