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ARM A32 Assembly Language: 32-Bit ARM, Neon, VFP, Thumb
Introduction
Assembly language is a low-level programming language that directly controls the hardware of a computer. It allows programmers to write code that is fast, efficient, and precise. However, assembly language is also complex, difficult, and error-prone. Therefore, it is usually used for specific tasks that require high performance or low-level access to the hardware.
ARM A32 Assembly Language: 32-Bit ARM, Neon, VFP, Thumb free download
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One of the most popular assembly languages is ARM A32 assembly language. It is used to program the 32-bit ARM processors that power many devices such as smartphones, tablets, embedded systems, and IoT devices. In this article, we will introduce the basics of ARM A32 assembly language and its advanced features such as Neon, VFP, and Thumb. We will also show you how to write, debug, and optimize ARM A32 assembly code using various tools and techniques.
What is ARM A32 Assembly Language?
ARM A32 assembly language is a common syntax for A32 and T32 instructions. A32 instructions are 32-bit instructions that operate on 32-bit registers and memory. T32 instructions are 16-bit or 32-bit instructions that operate on 16-bit or 32-bit registers and memory. Both A32 and T32 instructions are supported by all 32-bit ARM processors.
ARM A32 assembly language is also known as Unified Assembler Language (UAL). It supersedes earlier versions of both the A32 and T32 assembler languages. Code that is written using UAL can be assembled for A32 or T32 for any ARM processor. armasm faults the use of unavailable instructions .
Why learn ARM A32 Assembly Language?
Learning ARM A32 assembly language can be beneficial for several reasons:
It can help you understand how the hardware works and how the software interacts with it.
It can help you write code that is faster, smaller, or more energy-efficient than high-level languages.
It can help you debug or reverse-engineer code that is written in assembly language or compiled from high-level languages.
It can help you exploit or protect against vulnerabilities that are related to low-level operations.
It can help you learn other assembly languages or architectures that are similar to or derived from ARM.
How to write ARM A32 Assembly Language?
To write ARM A32 assembly language, you need a text editor, an assembler, a linker, a debugger, and an emulator or a device. Here are some examples of tools that you can use:
A text editor such as Notepad++, Visual Studio Code, or Sublime Text.
An assembler such as armasm (part of Arm Compiler), GNU Assembler (part of GNU Binutils), or Keil Assembler (part of Keil MDK).
A linker such as armlink (part of Arm Compiler), GNU Linker (part of GNU Binutils), or Keil Linker (part of Keil MDK).
A debugger such as Arm Development Studio (DS), Arm Keil µVision, or GNU Debugger (GDB).
An emulator such as QEMU, Arm Fast Models, or Arm Fixed Virtual Platforms (FVPs).
A device such as a Raspberry Pi, a BeagleBone, or an STM32 board.
The exact steps to write, assemble, link, debug, and run ARM A32 assembly code may vary depending on the tools and the target platform that you use. However, the general process is as follows:
Create a source file with the .s or .asm extension and write your assembly code using UAL syntax.
Use an assembler to convert your source file into an object file with the .o or .obj extension.
Use a linker to combine your object file with any libraries or startup code into an executable file with the .elf, .axf, or .bin extension.
Use a debugger to load your executable file into an emulator or a device and inspect or modify its behavior.
Use an emulator or a device to run your executable file and observe its output.
Basic Concepts
Registers and Memory
Registers are small storage units that are located inside the processor. They can store data or addresses and can be accessed quickly by the instructions. Memory is a large storage unit that is located outside the processor. It can store data or instructions and can be accessed slowly by the instructions.
ARM A32 assembly language supports 16 general-purpose registers (R0-R15) and 1 program status register (CPSR). The general-purpose registers can be used for various purposes such as holding data, pointers, arguments, results, etc. The program status register holds flags that indicate the state of the processor such as condition codes, interrupt masks, mode bits, etc.
The general-purpose registers have different names depending on their usage in different contexts. For example:
R0-R3 are used to pass arguments and return results in function calls.
R4-R11 are used to hold local variables and temporary values in functions.
R12 is used as an inter-procedure call scratch register or an intra-procedure call link register.
R13 is used as a stack pointer (SP) that points to the top of the stack.
R14 is used as a link register (LR) that holds the return address of a function call.
R15 is used as a program counter (PC) that holds the address of the next instruction to be executed.
The memory is organized into bytes (8 bits), halfwords (16 bits), words (32 bits), and doublewords (64 bits). The memory is also divided into regions such as code, data, stack, heap, etc. The memory can be accessed by using load and store instructions that transfer data between registers and memory. The memory can also be accessed by using literal pool instructions that load constants from memory into registers.
Data Types and Instructions
Data types are categories of data that have different sizes, formats, and meanings. Instructions are operations that manipulate data types according to certain rules. ARM A32 assembly language supports various data types and instructions such as:
Integer data types such as signed or unsigned bytes, halfwords, words, and doublewords.
Floating-point data types such as single-precision or double-precision numbers.
Vector data types such as 64-bit or 128-bit vectors of bytes, halfwords, words, or floating-point numbers.
Arithmetic instructions such as add, subtract, multiply, divide, etc.
Logical instructions such as and, or, xor, not, etc.
Shift and rotate instructions such as lsl, lsr, asr, ror, etc.
Move and compare instructions such as mov, mvn, cmp, cmn, etc.
Load and store instructions such as ldr, str, ldm, stm, etc.
Literal pool instructions such as adr, adrl, ldr= , etc.
The syntax of an instruction consists of four parts: mnemonic, condition code (optional), operands (one or more), and comment (optional). For example:
add r0,r1,r2 ; r0 = r1 + r2
The mnemonic indicates the operation to be performed. The condition code indicates when the instruction will be executed based on the flags in the CPSR. The operands indicate the source and destination registers or memory locations. The comment indicates any additional information for readability.
Addressing Modes and Labels
Addressing modes are ways of specifying the location of data or instructions in memory. Labels are names that are assigned to memory locations for convenience and readability. ARM A32 assembly language supports various addressing modes and labels such as:
Base register addressing: The simplest form of addressing is a single register that contains the full or absolute address of the data or instruction. For example:
LDR R0, [R1] ; R0 = memory[R1]
Offset addressing: An offset can be applied optionally to the base address to form a relative address. The offset can be either a constant or another register. This type of addressing is useful for accessing structs, arrays, or buffers. For example:
LDR R0, [R1,#12] ; R0 = memory[R1+12]
LDR R0, [R1,R2] ; R0 = memory[R1+R2]
Pre-indexed addressing: Pre-indexed addressing is like offset addressing, except that the base pointer is updated as a result of the instruction. This type of addressing is useful for pushing onto the stack. For example:
STR R0, [R1,#-4]! ; memory[R1-4] = R0; R1 = R1-4
Post-indexed addressing: Post-indexed addressing is like offset addressing, except that the base pointer is updated after the instruction. This type of addressing is useful for popping off the stack. For example:
LDR R0, [R1],#4 ; R0 = memory[R1]; R1 = R1+4
PC-relative addressing: PC-relative addressing uses the program counter (PC) as the base register and an offset as the operand. This type of addressing is useful for accessing constants or literals that are stored in a literal pool near the current instruction. For example:
LDR R0, [PC,#8] ; R0 = memory[PC+8]
Labels are symbolic names that are associated with memory addresses by using directives such as EQU, = , or : . Labels can be used to refer to data or instructions without knowing their exact addresses. Labels can also be used to define constants or macros that can be reused throughout the code. For example:
PI EQU 3.14159 ; define a constant named PI
ADD R0,R0,R0 ; double R0
B LOOP ; branch to LOOP label
Conditional Execution and Branching
Conditional execution and branching are ways of controlling the flow of the program based on certain conditions. Conditions are determined by the state of the flags in the CPSR register that are set or cleared by some instructions. ARM A32 assembly language supports various conditional execution and branching mechanisms such as:
Condition codes: Nearly all instructions can be executed conditionally by appending a two-letter suffix to the mnemonic that indicates the condition to be tested. For example:
ADDNE R0,R1,R2 ; R0 = R1 + R2 if Z == 0
BLE LOOP ; branch to LOOP label if (Z == 1) (N != V)
If-Then blocks: In Thumb-2 state, up to four consecutive instructions can be executed conditionally by using an IT instruction that specifies the condition and its variants. For example:
ITTE EQ ; if-then-then-else block with equal condition
MOVEQ R0,R1 ; move R0 to R1 if Z == 1
ADDEQ R0,R0,R2 ; add R0 and R2 to R0 if Z == 1
SUBEQ R0,R0,R3 ; subtract R0 and R3 from R0 if Z == 1
Compare and Branch instructions: In Thumb state, CBZ and CBNZ instructions can be used to compare a register against zero and branch on the result. For example:
CBZ R0,DONE ; branch to DONE label if R0 == 0
CBNZ R1,LOOP ; branch to LOOP label if R1 != 0
Neon and VFP Extensions
Neon and VFP are extensions to the ARM architecture that provide support for vector and floating-point operations. Neon is an advanced SIMD (Single Instruction Multiple Data) extension that can operate on 64-bit or 128-bit vectors of bytes, halfwords, words, or single-precision floating-point numbers. VFP is a floating-point extension that can operate on single-precision or double-precision floating-point numbers. Both Neon and VFP have separate register banks that can be accessed by using special instructions.
Neon and VFP extensions are optionally available only for the ARMv7-A and ARMv7-R architectures. Some Neon instructions are also available on systems that implement VFP extension without Neon. These are called shared instructions. Most VFP and the shared instructions are available in all versions of the VFP architecture. However, some features such as half-precision and fused multiply-add are only available in VFPv4.
Neon and VFP extensions can be useful for accelerating multimedia and signal processing algorithms such as video encoder/decoder, 2D/3D graphics, gaming, audio and speech processing, image processing, telephony, and sound. They can also be useful for scientific computing and high-performance computing applications that require high-precision floating-point arithmetic.
The syntax of a Neon or VFP instruction consists of four parts: mnemonic, condition code (optional), data type specifier (optional), and operands (one or more). For example:
VADD.F32 S0,S1,S2 ; S0 = S1 + S2
VLD1.8 D0-D1,[R0] ; load 16 bytes from memory into D0-D1
Thumb Instruction Set
Thumb is a 16-bit instruction set that is a subset of the 32-bit A32 instruction set. Thumb instructions can operate on 8-bit, 16-bit, or 32-bit data and registers. Thumb instructions can also interwork with A32 instructions by using special branch instructions. Thumb instruction set is designed to provide better code density and lower memory footprint than A32 instruction set.
Thumb instruction set is available in all ARMv4T and later architectures. However, different versions of Thumb instruction set have different features and capabilities. For example:
Thumb-1 is the original version of Thumb instruction set that has limited functionality and performance compared to A32 instruction set.
Thumb-2 is an enhanced version of Thumb instruction set that introduces 32-bit instructions and provides almost the same functionality as A32 instruction set.
Thumb-2EE is a variant of Thumb-2 instruction set that supports dynamic code generation and execution.
Thumb instruction set can be useful for applications that have limited memory space or bandwidth, such as embedded systems, IoT devices, or mobile devices. Thumb instruction set can also be useful for applications that require fast context switching or interrupt handling, such as real-time systems or operating systems.
The syntax of a Thumb instruction consists of four parts: mnemonic, condition code (optional), data type specifier (optional), and operands (one or more). For example:
ADDS R0,R1,R2 ; R0 = R1 + R2
BNE LOOP ; branch to LOOP label if Z == 0
Macros and Directives
Macros and directives are special instructions that are processed by the assembler rather than the processor. Macros are blocks of code that can be defined and invoked by using a name. Directives are commands that control the assembly process or generate output. ARM A32 assembly language supports various macros and directives such as:
MACRO and MEND directives: These directives are used to define and end a macro. A macro can have a name, parameters, local labels, and a body of code. A macro can be invoked by using its name and passing arguments if any. For example:
MACRO ; define a macro named SWAP
$r1,$r2 ; parameters are two registers
XOR $r1,$r1,$r2 ; swap the values of the registers
XOR $r2,$r1,$r2
XOR $r1,$r1,$r2
MEND ; end the macro definition
SWAP R0,R1 ; invoke the macro with R0 and R1 as arguments
IF and ENDIF directives: These directives are used to conditionally assemble a block of code based on an expression. The expression can be evaluated at assembly time or run time. For example:
IF :DEF:DEBUG ; if DEBUG symbol is defined
BL PRINT ; call PRINT function
ENDIF ; end the conditional block
AREA directive: This directive is used to define a section of code or data with a name, attributes, and alignment. A section can contain instructions, data, labels, or other directives. For example:
AREA CODE,CODE,READONLY ; define a code section named CODE
START MOV R0,#0 ; first instruction of the section
B END ; last instruction of the section
Debugging and Optimization
Debugging and optimization are processes that improve the quality and performance of the assembly code. Debugging is the process of finding and fixing errors or bugs in the code. Optimization is the process of making the code run faster, use less memory, or consume less power. ARM A32 assembly language provides various tools and techniques for debugging and optimization such as:
Breakpoints and watchpoints: Breakpoints and watchpoints are features that allow the debugger to stop the execution of the code at a specified point or when a specified condition occurs. Breakpoints can be set on instructions or labels. Watchpoints can be set on registers or memory locations. For example:
BREAKPOINT 0x1234 ; set a breakpoint at address 0x1234
WATCHPOINT R0 == 0 ; set a watchpoint when R0 equals 0
Trace and profiling: Trace and profiling are features that allow the debugger to record and analyze the execution history of the code. Trace can show the sequence of instructions executed, the values of registers and memory locations, and the timing information. Profiling can show the frequency and duration of functions, loops, or instructions executed. For example:
TRACE ON ; start tracing
PROFILE ON ; start profiling
Assembly directives: Assembly directives are commands that control the assembly process or generate output. Some assembly directives can be used for debugging or optimization purposes. For example:
ASSERT R0 != 0 ; generate an error if R0 equals 0
OPTIMIZE 2 ; enable level 2 optimization
Conclusion
In this article, we have introduced the basics of ARM A32 assembly language and its advanced features such as Neon, VFP, and Thumb. We have also shown you how to write, debug, and optimize ARM A32 assembly code using various tools and techniques. We hope that this article has given you a solid foundation for learning and using ARM A32 assembly language for your projects.
ARM A32 assembly language is a powerful and versatile low-level programming language that can help you to understand how the hardware works and how the software interacts with it. It can also help you to write code that is faster, smaller, or more energy-efficient than high-level languages. However, ARM A32 assembly language is also complex, difficult, and error-prone. Therefore, it is usually used for specific tasks that require high performance or low-level access to the hardware.
If you want to learn more about ARM A32 assembly language or other aspects of the ARM architecture, you can refer to the following resources:
The official documentation of the ARM architecture and tools at https://developer.arm.com/.
The online courses and tutorials on ARM assembly language at https://www.coursera.org/learn/arm-assembly-language or https://www.udemy.com/course/arm-assembly-language-from-ground-up/.
The books and articles on ARM assembly language such as Modern Arm Assembly Language Programming by Daniel Kusswurm or ARM Assembly Language: Fundamentals and Techniques by William Hohl and Christopher Hinds.
FAQs
Here are some frequently asked questions about ARM A32 assembly language:
What is the difference between A32 and T32 instruction sets?
How can I switch between A32 and T32 instruction sets?
What are the advantages and disadvantages of using Ne