MASM Assembly

Introduction

The Microsoft Macro Assembler (MASM) provides several advantages over inline assembly. MASM contains macro features that include loops, arithmetic, and text string processing. MASM gives you greater control over your hardware such as CPU and memory.

MASM is generally used for programming firmware, developing operating systems, and programming at system level.

Syntax

There are several assembly syntax types, the two most important are:

Intel Syntax - Widely used in Windows Operating Systems. Friendly look.

AT&T Syntax - Generally used in Unix Systems. Default for GDB debugger.

The Art of 64-Bit Assembly, Volume 1: x86-64 Machine Organization and ProgrammingAmazon.com

Introduction to Registers

CPU Registers are small, high-speed storage locations within the CPU used to store data and addresses during the execution of instructions. Registers are the single place where mathematical functions (additions, multiplication, subtractions) can be carried out. Registers often hold pointers that refer to the memory.

Types of Registers

CPU registers can mainly be classified into 4 different categories.

• General Purpose Registers

• Segment Registers

• Special purpose application-accessible registers

• Special Purpose Kernel-Mode Registers

In this page we will only go over general purpose registers since their commonly used by programmers.

General Purpose Registers

Used to store temporary data. It's content can be accessed by assembly programming. Numbered: R0, R1, R2,...Rn-1.

Windows x86 Architecture

x86 Architecture - Windows driversMicrosoftLearn

In Windows x86 the general purpose registers look like this:

x86 Register Structure

The following diagram shows the first two registers. EAX & EBX.

Windows x64 Architecture

x64 Architecture - Windows driversMicrosoftLearn

In Windows x64 the general purpose registers look like this:

x64 Register Structure

The following diagram shows the first two registers. RAX & RBX.

IMPORTANT: It's important to note that structure of these registers are not independent. They are arranged in a hierarchical structure. Where registers of larger size overlay smaller ones. RAX (64 bit) overlays EAX (32 bit), which in turn overlays the 16 bit registers (AX & AH/AL).

**Therefore, modifying the value of RAX will change the value of EAX, and therefore the values of AX. **The hierachical relationship implies that changing the values of higher registers effects the value of lower registers, vice versa.

For example, modifying the value of BL will impact the value of BX, which will then influence the value of EBX, and subsequently modify the value of RBX.

Volatile vs Non-Volative Registers

During a function or procedure call (assembly functions are called procedures), certain registers automatically change value. These are called non-volatile registers.

Non-Volatile RegisterTechopedia

Common Non-Volatile registers:

• x64 Systems - RSP, RSI, RDI, RBP, RBX, R12-15.

• x86 Systems - EBX, EBP, ESI, EDI, R12-R15D.

On the other hand, volatile registers do not need to be saved across a function/procedure call:

• x64 Systems - RCX, RAX, RDX, R8-11.

• x86 Systems - ECX, EAX, EDX, R8-11D.

IMPORTANT: Whenever a value of a non-volatile register is changed by the routine (procedure), the old value has to be saved on the stack prior to changing the register and that value has to be restored before returning.

RSP & RIP Registers

The RIP/EIP register is a special-purpose register that holds the memory address of the next instruction being executed. The processor automatically increments the RIP/EIP register after executing each instruction.

The RSP/ESP register is called the stack pointer register. It holds the memory address of the top of the stack. (The stack is a memory region that's used to store temporary data & function call information. The RSP/ESP keeps track of the stacks current location).

RDI, RSI, RDX Argument Registers

The general purpose registers rdi, rsi, rdx, rcx, r8, and r9 are typically used for parameter passing. These registers are known as "Arguments registers", they hold values that are passed to a function.

int result = add(3,6);

In the example above, the values 3 and 6 would be passed to the add function using registers. rdi might hold 3 and rsi might hold 6.

RFLAGS Register

The RFLAGS (Register Flags) is a special-purpose register that contains several status and control flags that are used by the processor to control program execution.

64-bit machines the RLAG is 64 bits in size, 32-bit: 32 bits. The register comprises several single-bit values, where each bit corresponds to a single flag. A flag is set to 1 when activated and 0 when deactivated.

IMPORTANT: The majority of RFLAGS flags are reserved for kernel-mode functions, they are limited to general users.

The relevant flags are explained below:

• Carry Flag (CF) -- This flag is set when an arithmetic operation generates a carry or borrow. It is also used in bitwise operations, where it indicates whether the result of the operation has a carry-out from the most significant bit.

• Parity Flag (PF) - This flag is set when the least significant byte of the result of an arithmetic operation has an even number of set bits.

• Zero Flag (ZF) - This flag is set when the result of an arithmetic operation is zero.

• Sign Flag (SF) - This flag is set when the result of an arithmetic operation is negative.

• Overflow Flag (OF) - This flag is set when an arithmetic operation generates a signed overflow, meaning that the result is too large to be represented in the available number of bits.

MASM Assembly Program Structure

Here is a typical MASM program, the semicolon ; denotes a comment.

; Data section: contains variable and memory values, adding this section is optional 
; Variables can be declared below the ".data" directive
.data


; Code section: contains the assembly code/functions
; Assembly functions can be declared below the ".code" directive
.code


; MASM function declaration
main PROC ; Start of function "main"
     
      ; Assembly code of "main"
      
      ret ; Return from "main"     
main ENDP ; End of function "main"    


; The "end" directive marks the end of the source file
end

Declaring Variables

Variables must be declared within the .data section of the program.

VarName directive VarValue

VarName is the variable name you want.

Here is a list of possible directives:

• word - Unsigned 16-bit value (word).

• sword - Signed 16-bit integer value.

• dword - Unsigned 32-bit value (double word).

• sdword - Signed 32-bit integer value.

• qword - Unsigned 64-bit value (quad word).

• sqword - Signed 64-bit integer value.

• oword - 128-bit value (octal word).

• tbyte - Unsigned 80-bit value.

• real4 - 32-bit floating point value.

• real8 - 64-bit floating point value.

• real10 - 80-bit floating point value.

• byte - Unsigned 8-bit value.

• sbyte - Signed 8-bit integer value.

Declaring Value:

VarValue is our value:

WordVariable      word         2
sWordVariable     sword       -2
FloatVariable     real8       3.1

Declaring value as Hexadecimal:

We can initialize a value with hexadecimal using the h suffix.

DwordVariable     dword       10h         ; this is 10 in hex, which is 16 in decimal

Declaring Strings:

Strings are declared using byte directive

StringVar  byte 'This is a string', 0    ; we add "0" to null-terminate the string 

The MASM assembler interprets the above string as an array of hexadecimal characters. We can incorprate a new line character as hexademical: 10

StringVar byte 'This is a string with a new line', 10, 0  ; "10" represents the new line character and is equal to 16 in decimal format

Since the byte directive in MASM assumes that it is dealing with hexadecimal characters, it is unnecessary to include the h suffix to represent the value of 10.

Assembly Instructions

The following section goes over common Assembly instructions. A full list can be found:

mov

The mov instruction is the most frequently used instruction in assembly. As the name suggests, it is used to move data between registers or memory locations.

mov destination, source

Both destination or source can be a general purpose register or memory variable. The mov instruction is limited to:

• Only one of the source and destination operands can be a memory variable.

• Both the source and destination operands must be of the same size. Mixing different operand sizes within a single mov instruction will result in a compilation error.

Here is a list of all legal mov instructions

mov rax, 1234     ; move the value 1234 into the RAX register
mov rax, rbx      ; move the value in the RBX register into the RAX register

mov al, 5h        ; move the value 0x05 into the AL register
mov [ebx], al     ; move the value in AL to the memory location pointed to by the EBX register

In assembly language, square brackets [] are utilized to indicate indirect memory access. It points to the source of the memory location. Similar to pointers in C.

add & sub

add & sub insturctions adds and subtracts to operands. They share the same syntax.

add destination, source ; destination = destination + source
sub destination, source ; destination = destination + source

add rax, rbx      ; add the value in RBX to the value in RAX and store the result in RAX
add rax, [rcx]    ; add the value in the memory location at RCX to the value in RAX and store the result in RAX
add [rax], 10     ; add the value 10 to the memory location at RAX and store the result in that memory location

mov al, 12h       ; move the value 0x12 into the AL register
mov bl, 5h        ; move the value 0x05 into the BL register
sub al, bl        ; subtract the value in BL from the value in AL and store the result in AL. AL's value is now '13'

call & ret

Procedure calls are made with the call instruction. The ret instruction is then used to return execution back to the caller, which serves a similar purpose as C/C++.

call  ProcedureName  ; ProcedureName is the function name we call.

The ret instruction does not require any parameters / operands. It does not return any value, it's purpose is to indicate that the current function is finished executing. The address that is returned from ret is determined by the value at the top of the stack.

ret

Example code

Here is an example of ret & call instructions.

.code

DummpProc     PROC
      mov rcx, 3        ; dummy code
      add rbx, 2
      sub esi, 1
      ret               ; return execution back to "main"
DummpProc     ENDP


main          PROC
      call DummpProc    ; calling "DummpProc"
      ret               ; function "main" is terminated
main          ENDP

end

lea

The Load Effective Address (lea) instruction returns the memory address of a location and load it into a register, without actually accessing the memory itself. It's essentially the & address-of operator in C/C++.

lea reg64, source  ; reg64 represents a 64-bit general-purpose register

Where reg64 (the destination operand) represents any 64-bit general-purpose register that will hold the address of the source memory location.

StringVar byte 'String Variable', 0       ; A dummy string variable 
lea rcx, StringVar                        ; Load the address of the StringVar variable into RCX. RCX is now equal to &StringVar[0]

and, or, xor, not

The logical operators and, or, xor, and not are all used to perform logical operations on bits.

and

The and instruction performs a bitwise and operation between two operands and stores the result in the destination operand.

and destination, source

or

The or instruction performs a bitwise or operation between two operands and stores the result in the destination operand.

or destination, source

xor

The xor instruction performs an exclusive OR operation between two operands and stores the result in the destination operand. One common use of the xor instruction is to clear a register, which is achieved by XORing the register with itself. The syntax of the xor instruction is as follows:

xor destination, source

not

The not instruction performs a bitwise not operation on the operand and stores the result in the destination operand. The syntax of the not instruction is as follows:

not destination

jmp

The jmp instruction, jumps to the destination operand. It can be a memory address, register, or a label. It's used for unconditional branching or jumping.

jmp   destination       ; Where 'destination' is where to jump 

NOTE: In assembly language, a label is a name given to a specific location in the program's code, which is usually defined using a colon (:) at the end of a name or identifier.

Example jmp

.code

main PROC
      add eax, 2              ; dummy code
      xor ax, 5
      mov bx, ax
      jmp LabelName           ; Jump to execute 'LabelName' 
      mov eax, 100            ; These instructions won't get executed
      mov ebx, 100
LabelName:
      xor eax, eax            ; LabelName's code
      sub ebx, 2      
      ret
main ENDP

end

jz & jnz

jz and jnz instructions are conditional jump instructions, which allow for conditional execution of code. These instructions work by checking a specified flag in the RFLAGS register.

jz, which stands for "jump if zero", jumps if the zero flag is set (1), while jnz ("jump if not zero") executes the jump if the zero flag is clear (0). There are many other conditional jump instructions:

• jc Jump if Carry - Executes the branch if the Carry Flag is set (1).

• jnc Jump if Not Carry - Executes the branch if the Carry Flag is not set (0).

• jo Jump if Overflow - Executes the branch if the Overflow Flag is set (1).

• jno Jump if Not Overflow - Executes the branch if the Overflow Flag is not set (0).

• js Jump if Sign - Executes the branch if the Sign Flag is set (1).

• jns Jump if Not Sign - Executes the branch if the Sign Flag is not set (0).

• je Jump if Equal - Executes the branch if the Zero Flag is set (1).

• jne Jump if Not Equal - Executes the branch if the Zero Flag is not set (0).

• ja Jump if Above - Executes the branch if the left operand is greater than the right operand.

• jae Jump if Above or Equal - Executes the branch if the left operand is greater than or equal to the right operand.

• jb Jump if Below - Executes the branch if the left operand is less than the right operand.

• jbe Jump if Below or Equal - Executes the branch if the left operand is less than or equal to the right operand.

cmp

The cmp instruction or "compare" is the most useful instruction to execute prior to a conditional jump instruction.

cmp First, Second

The cmp instruction subtracts the second operand from the first operand and sets the condition code flags based on the result of the subtraction. NOTE: It does not store the difference back into the first (destination).

The following examples demonstrate how cmp can set a flag's value based on the value of its operands.

• If the first operand is greater than the second operand, the Carry flag is cleared and the Sign flag is set if the result is negative.

• If the second operand is greater than the first operand, the Carry flag is set and the Sign flag is cleared.

• If the two operands are equal, the Zero flag is set and the Carry and Sign flags are cleared.

The cmp instruction is usually used in conjuction with a jmp. Here's an example of dissembled C code:

#include <stdio.h>

int main() {

	int i = rand();
	// if "i" is not equal to 10
	if (i != 10) {
		printf("i != 10 \n");
	}

	return 0;
}

The following assembly code shows a je instruction being found directly below a cmp instruction.

push & pop

The push and pop instructions are used to manipulate the stack.

push takes a value from a register and pushed it onto the top of the stack.

push Source

pop takes the value at the top of the stack and pops it off, storing it in the destination register or memory location.

pop Destination

leave

The leave instruction is used to clean up or exit a subroutine or function.

When executed, it first moves the value of the base pointer register (RBP) to the stack pointer register (RSP). It then pops the value of the base pointer register from the stack, restoring it to its previous value.

Essentially, the leave instruction performs the same task as the following instructions:

mov rsp, rbp
pop rbp

Memory Access Specifiers

In assembly, memory access specifiers are used to determine the size and the type of data being accessed in memory. These specifiers act like type-casting in a programming language.

The most commonly used Memory Access Specifiers are:

Quadword Pointer - qword ptr

A quadword pointer is used to access a 64-bit data value stored in memory. It is specified using the qword ptr specifier. For instance, if you want to access a 64-bit integer value stored in a particular memory location, you can use the qword ptr specifier with the mov instruction. Here are two examples:

mov rax, qword ptr [rbx]         ; Example 1
mov rax, qword ptr [rsp + 32h]   ; Example 2

In the first example, the 64-bit integer value stored at the memory location pointed to by the rbx register is accessed using the qword ptr specifier with the mov instruction. In the second example, the qword ptr specifier is used with the mov instruction to access the 64-bit integer value stored at an offset of 32h bytes from the rsp register.

Doubleword Pointer - dword ptr

A doubleword pointer is a memory addressing mode that specifies the size of 32-bit data in memory. It is used when manipulating data stored in memory, particularly 32-bit integer values. To access a 32-bit integer value stored at a specific memory location, the dword ptr specifier should be used in the instruction, as shown in the following examples:

mov dword ptr [ebx], 12345678	; Example 1: stores a 32-bit integer value in memory
mov eax, dword ptr [edx + 4]	; Example 2: loads a 32-bit integer value from memory into the eax register

Byte Pointer - byte ptr

A byte pointer is used to indicate the size of 8-bit data in memory. To access a single byte of data stored at a specific memory location, the byte ptr specifier is used.

mov al, byte ptr [edx + 2]	; Example 1	
mov byte ptr [ebx + 8], 55h	; Example 2	

Calling Functions

Calling functions in assembly can happen a couple ways:

Calling assembly function via call

1.) Calling the assembly function via call instruction with ret used to return the caller.

call power

power:
        push ebp                # save old base pointer
        mov esp, ebp           # make stack pointer the base pointer

Calling the assembly function from C

We can import an assemly function to a C file. The function prototype is defined with the extern keyword. This informs the compiler that the function is already in another file, such as an .asm file.

Example of calling assembly from C:

/*
      main.c file
*/

#include <stdio.h>

extern void SimpleAsmFunc(); // SimpleAsmFunc's prototype. Parameters and function return data type is covered in a later section

int main (){
      printf("[i] Calling 'SimpleAsmFunc' ... ");
      SimpleAsmFunc();
      printf("[+] Done");
      return 0;
}

; The asm file that includes the definition of 'SimpleAsmFunc'

.code

SimpleAsmFunc PROC
      xor rcx, rcx      ; SimpleAsmFunc's code
      add rcx, 2
      ret
SimpleAsmFunc ENDP

end

Calling a C function from within an assembly file.

To do this, the assembly code must first declare the C function using the externdef directive. This tells the MASM assembler that the function is in another file.

externdef symbol_name:type

Here the externdef is the name of the function and the type specifies the function type.

externdef foo:proc	; This will tell MASM that "foo" is a procedure

Example calling C code from assembly:

/*
      main.c file
*/

#include <stdio.h>

// Dummy C function
void SimpleCFunc() {

	int i = 100;
	i = i * (i + 7) >> 3;
	i += i/2;

	if (i > 100)
	   i -= 20;
        else
	   i += 20;
}


int main() {
	// You can port "AsmFunc" here and call it
	return 0;
}

; The asm file that calls 'SimpleCFunc'

externdef SimpleCFunc:proc 	; Using externdef to declare "SimpleCFunc" as a procedure defined in an other file

.code 

AsmFunc PROC

      call SimpleCFunc		; Calling SimpleCFunc
      ret

AsmFunc ENDP

end

Passing Parameters

The first four parameters (if they exist) are passed through the registers RCX, RDX, R8, and R9.

NOTE: If a procedure requires more than four parameters, they are pushed onto the stack. These parameters are known as stack parameters, and the stack must be 16-byte aligned to accommodate them.

IMPORTANT: The first stack parameter (5th procedure parameter) is located at a specific offset from the rsp register, depending on the function's calling convention. In a 64-bit MASM function, the fifth parameter is usually located at an offset of [rsp + 40].

Example passing parameters:

AsmFunc11Parms PROC

    ; RCX => Parm1
    ; RDX => Parm2
    ; R8  => Parm3
    ; R9  => Parm4

    mov rax, qword ptr [rsp + 40]  ; Parm5
    mov rax, qword ptr [rsp + 48]  ; Parm6
    mov rax, qword ptr [rsp + 56]  ; Parm7
    mov rax, qword ptr [rsp + 64]  ; Parm8
    mov rax, qword ptr [rsp + 72]  ; Parm9
    mov rax, qword ptr [rsp + 80]  ; Parm10
    mov rax, qword ptr [rsp + 88]  ; Parm11

    ret

AsmFunc11Parms ENDP

Calling AsmFunc11Parms from C is done below

#include <Windows.h>

extern int AsmFunc11Parms(PVOID Parm1, PVOID Parm2, PVOID Parm3, PVOID Parm4, PVOID Parm5, PVOID Parm6, PVOID Parm7, PVOID Parm8, PVOID Parm9, PVOID Parm10, PVOID Parm11);

int main() {
	AsmFunc11Parms(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11);
	return 0;
}

Returning Value

When assembly returns a value it is stored in the RAX register. Before executing the ret instruction, the procedure saves the value inside the RAX register. Allowing the function to return a value.

The following AddtwoNumbers procedure, takes two parameters, to return their sum.

AddtwoNumbers PROC
    mov rax, rcx    ; Moving the 1st parmeter to RAX  
    add rax, rdx    ; Add the 2nd parmeter to the value in RAX
    ret             ; return (RAX here is RCX + RDX)
AddtwoNumbers ENDP