Difference between revisions of "Assembly"

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This article needs immediate attention, and is in desperate need of more content. There should be zero full assembly programs on this page, however examples for most common instructions.

Introduction

An assembler is a program that compiles relatively human-readable operations into instructions that can be directly interpreted by the processor. Assembly Language can be seen as a "mnemonic" for machine instructions - it consists of words that are easier for humans to remember than machine language instructions - for example, int for the interrupt operation rather than 0xcd. The assembler produces compiled "objects", translating these mnemonics into machine code - also known as opcodes or as binary code in popular culture. By convention, many assembly programmers use the .s extension for assembly code and the .o extension for compiled objects.

On Linux platforms, as is the standard assembler.

A linker is a program that combines the compiled assembly objects into a binary. ld is the standard linker on Linux platforms.

The way in which assembly code is assembled and linked is similar to the way in which higher-level compilers operate. Compilers such as GCC are often simply wrappers for an assembler and a linker that perform both functions dynamically. GCC assembles code into a number of object files based on the rules of the C programming language, then links them into a flat binary of the opcode sequences created at the assembly stage - this flat binary is encoded according to the executable format of the Operating System, which is why the same code will need to be recompiled for different operating systems in order for it to run cross-platform. At runtime, this binary interacts with RAM and hardware gates on the CPU according to the rules that the Operating System and CPU architecture follow, performing the desired function of the program.

Binary

Main article: Bitwise Math

• nybble

A nybble is an uncommon unit of memory equivalent to 4 bits of data. This unit is not used very often, especially in 32bit and 64bit programming. A nybble can also be thought of as a half of a byte of data.

• byte

A byte is the base unit of every type of data. It is equivalent to 8 bits of data or 2 nybbles and is one of the most used data types in programming. Within assembly there are three main kinds of instructions, one that deals with bytes, one that deals with dwords, and one that deals with qwords. Depending on which instructions you use with which data types there would be padding of null bytes added to it. For instance if you used the instruction pushl 0x0a the data that is actually being pushed to the stack would be 0x0000000a where as if you used the proper instruction(pushb) for pushing bytes to the stack it would simply push 0x0a by itself. The same would happen if you tried to push a byte to the stack using a qword instruction, except that instead of pushing 3 padded null bytes with your data it would push 7.

• word

A word is a data type which is 2 bytes in size or 16 bits. There are fewer instructions that work with this size directly. Most instructions either accept 1 byte, 1 dword, or 1 qword of data to operate upon.

• dword

A dword is the second most common data type compared to the byte. A dword consists of 4 bytes and is equivalent to 32 bits of data. Dwords are also known as longs and have a large array of instructions that use and manipulate data using this size. The only instructions that are capable of padding dwords with null bytes are the ones that use qwords as data input. Dwords are also the maximum size of memory addresses on the 32bit operating systems.

• qword

This is the last data type that is used in assembly programming. It is 8 bytes in size and is equivalent to 64 bits of data. No current instructions can cause null byte padding for qwords on the 64bit operating systems. Qwords are also the maximum size of memory addresses on the 64bit operating systems.

Number handling

Two's complement is the mathematics principle behind the computer's ability to track positive and negative numbers.

• signed integers

Signed values are required to represent negative numbers. Most languages by default assume values are signed. The range of numbers it can assign extends from -1 downwards, depending on the data type. The method by which signed values are handled is known as two's complement. For a value signed with two's complement, the maximum number of possible permutations of the space allocated for the value is divided in two, and of the highest value, half is used to represent negative numbers (i.e. from -1 to -1 * (highest_value/2)) while the other half is used for positive numbers (i.e. from 0 to highest_value - 1). For example, with a signed dword, 00000000 is equal to 0, 00000001 is equal to 1 as would be expected. However, in addition 1111111 is equal to -1, 11111110 is equal to -2, and so on until it reaches the halfway point, which is the highest negative value.

• unsigned integers

Despite not being able to assign negative numbers, unsigned values are particularly advantageous for positive ranges. The memory that would have been assigned to the negative range is instead added to the positive range (twice as many positive numbers). Unsigned numbers work exactly as would be expected when counting - 0001 is 1, 0010 is 2, 0011 is 3 and so on until the all the bits are filled, which is the maximum number that can be represented with that many bits. For example, since 2 bits have 4 possible permutations - 00, 01, 10 and 11 - it would be possible to count from 0 to 3 with 2 bits. The key feature is that they can only represent 0 and positive numbers.

• example

The C datatype short int denotes an integer stored in 2 bytes The unsigned short int is able to represent numbers between 0 and 65,535.

The signed short int, on the other hand, is able to represent numbers between -32,768 and 32,767.

Data storage

Register

A location where memory can be stored temporarily. A register has the bit-width of a cpu's bit description. So for 32 bit systems, a register is 32 bits (4 bytes or a doubleword, also called long) whereas on a 64 bit system a register is 64 bits in length (8 bytes or a qword).

A register is a location where memory can be stored temporarily. It can be considered to be a sort of basic variable, which can hold any value that the processor stores in it. The difference between registers and the variables of higher-level programming languages is twofold: firstly, registers are limited in size. The reigsters of a processer will have the bit width of the cpu's bit description - 32-bit processors have 32-bit (4-byte) registers, whereas 64-bit systems have a 64-bit (8 byte) register.

The other difference is that registers have both a limited number and in many cases a specific purpose. Registers cannot be defined at the whim of the programmer because unlike variables, a register is not simply a region of allocated memory. They occupy physical space and have a limited number. Some registers are open to use by programs and are "general purpose registers" - they can be loaded with any information and are often used by the operating system to hold arguments to function calls. Other registers such as the instruction pointer, which holds the address of the next instruction that is being executed, are reserved for cpu function.

Pointer

A pointer is simply a hexadecimal memory address or offset that points to another location in memory. Pointers have many uses within programming, namely that it is easier to work with a pointer to some data than to have to copy and recopy the data each time it is referenced. One area in which pointers are used is the instruction pointer register, which contains a pointer to the next instruction to be executed.

Sub-Register

A subregister is a portion of a register that is always divisible by a whole byte in size. They are often used where an entire 4 bytes (or 8 bytes) is not necessary to complete an operation. One area in which sub-registers are used is in writing Shellcode which is intended to be injected into a string - subregisters are used to avoid the inclusion of null-bytes, which are interpreted by most systems as the string terminator.

CPU Flag Registers

A flag register is a collection of bits used by a processor to indicate various on-off states of different conditions. Each bit corresponds to its own flag, which can be set to etiher on or off by instructions, and can be referenced by a program (or manually set/unset by a program) in order to make decisions based on the result of an operation.

For example:

• PF - the parity flag.

This indicates whether the number of bits of the previous result is even or odd.

• ZF - the zero flag.

This indicates, when set, that the result of the previous operation was zero.

• CF - the carry flag.

This indicates, when set, that bit rotation across datasets larger than the cpu register's bit-width will be successful. Without this, data will be rotated in increments of the bit-width of a register for the cpu of the given architecture.

Architecture-specific Registers and Sub-Registers

x86

The following registers and subregisters are specific to 32 bit x86 instruction sets. The 32 bit registers are also found in conjunction with 64 bit specific registers in 64 bit x86 instruction sets.

32 bit general purpose registers

eax

eax is part of a family of several general purpose registers. The other members of this family are ebx, ecx and edx. The general purpose registers are used for general operations, serving as temporary storage for data used by the program. Many functions also use the general purpose registers to hold arguments - for example, on Linux, the kernel interrupt stores the interrupt code in eax, and any additional arguments in ebx, ecx and so on.

As can be seen from the diagram above, eax is split into several sub-registers. The least significant 2 bytes of eax forms another subregister called ax. This in turn, is split into 2 one-byte sub-registers. The most significant byte of ax is ah, while the least significant byte is al.

ebx

ecx

edx

esi

edi

eip

The eip register is the 32-bit instruction pointer used to represent the location of the current process counter in memory.

esp

ebp

64 bit general purpose

The following registers and subregisters are specific to 64bit x86 instruction sets, they include the 32 bit registers found in 32 bit x86 instruction sets.

rax

rax is the 64-bit equivalent of eax. Being a 64-bit register, it is 8 bytes rather than 4 but apart from this it can be treated as a normal register. The relationships that apply to eax still apply to rax - there are other 64-bit registers in the form of rbx, rcx and rdx. Keep in mind that in AT&T syntax, q is the operation suffix for working with 64-bit operands.

The lowest-order 4 bytes of rax are taken up by eax, which itself is divided as mentioned above under 32-bit general purpose registers.

rbx

rcx

rdx

rsi

rdi

rbp

rsp

rip

The rip instruction pointer register does not break down at all. Memory addresses on a 64-bit system can go as high as 8 bytes, although most systems do not have anywhere near enough RAM to break above 6.

%r8

64-bit processors include several additional registers not found on a 32-bit processor. The registers, labelled from 8 to 15, function as general purpose registers. The way they break down is slightly different. The letter appended to a subregister matches the initial of the size of the register. For example, for %r8: %r8d is the 4-byte (DWORD) subregister, while %r8b is the 1-byte (BYTE) subregister.

%r9

%r10

%r11

%r12

%r13

%r14

%r15

Streaming SIMD Extensions (SSE)

MMX

MMX defines eight registers, called MM0 through MM7, and operations that operate on them. Each register is 64 bits wide and can be used to hold either 64-bit integers, or multiple smaller integers in a "packed" format: a single instruction can then be applied to two 32-bit integers, four 16-bit integers, or eight 8-bit integers at once.

SSE

SSE2

SSE3

SSSE3

SSE4

XOP, FMA4, CVT16

AVX

AVX2

AVX512

Memory Addressing

Stack Pointer

Commonly known as the ESP register in x86 Assembly, the stack pointer is a register that contains the location of the top of the stack. Every time a push instruction is used to add data to the top of the stack, the number of bytes added is subtracted from esp (remember, the stack grows downwards) so that it points to the new top of the stack. Similarly, esp increments when data is popped from the stack.

Segments

When first writing an assembly application, a few declarations appear in the header:

.section .data
.section .bss
.section .text

• These areas are mapped to the data segment, the stack segment, and the code segment frame registers during execution of an executable file by the operating system, respectively.

.section .data

.section .bss

.section .text

• These are used as prefixes for other addresses:

%ds:(%reg)

%ss:(%reg)

%cs:(%reg)

Instruction Pointer

Commonly known as the EIP register in x86 Assembly, the instruction pointer is a register that holds the address to the next instuction. When a return instruction is executed, the instruction pointer derives its address from the return address, which exists on the stack. Likewise, when the call instruction is used to transfer execution to a function, the current value of eip (which holds the address of the instruction after the call function) is pushed to the stack so that there is a return address to pop when the return instruction executes and execution resumes from the main thread.

The instruction pointer is very useful in executing Shellcode within overflows - if eip is hijacked, the execution of the program can be arbitrarily controlled.

Base Pointer

Commonly known as the EBP register in 32 bit x86 Assembly, the base pointer is generally used to find local variables and parameters on the stack. It is often used during function calls to "save" the position that esp pointed to when the function was called, so that additional space can be used on the stack without losing it's place.

 
movl %esp, %ebp
subl $4, %esp