The aim of this project is to get familiar with assembly language.
brew install nasmor :
curl https://www.nasm.us/pub/nasm/releasebuilds/2.14.03rc2/macosx/nasm-2.14.03rc2-macosx.zip -o nasm.zip && unzip nasm.zip && rm nasm.zip && mv nasm* nasm && export PATH=~/nasm:$PATHfull tutorials:
https://cs.lmu.edu/~ray/notes/nasmtutorial/
https://www.tutorialspoint.com/assembly_programming/index.htm
others:
https://plantation-productions.com/Webster/www.artofasm.com/Linux/HTML/MemoryAccessandOrg.html
https://en.wikipedia.org/wiki/Kernel_(operating_system)#System_calls
https://www.cs.virginia.edu/~evans/cs216/guides/x86.html
http://cs.brown.edu/courses/cs033/docs/guides/x64_cheatsheet.pdf
https://www.cs.uaf.edu/2015/fall/cs301/lecture/09_16_stack.html
https://docs.oracle.com/cd/E26502_01/html/E28388/eoiyg.html
http://www.egr.unlv.edu/~ed/assembly64.pdf
https://www.ic.unicamp.br/~pannain/mc404/aulas/pdfs/Art%20Of%20Intel%20x86%20Assembly.pdf
https://medium.com/@jain.sm/invoking-a-system-call-via-assembly-84c9f8832105
https://stackoverflow.com/questions/47834513/64-bit-syscall-documentation-for-macos-assembly
Assembly language is a low-level programming language for a computer, or other programmable device . A low-level language is a type of programming language that contains basic instructions recognized by a computer.
NASM is line-based. Most programs consist of directives followed by one or more sections. Lines can have an optional label. Most lines have an instruction followed by zero or more operands.
The data section is used for declaring initialized data or constants. This data does not change at runtime. You can declare various constant values, file names, or buffer size, etc., in this section.
The text section is used for keeping the actual code. This section must begin with the declaration global _start, which tells the kernel where the program execution begins.
The bss section is used for declaring variables.
Programs written in assembly language consist of a sequence of source statements. Each source statement consists of a sequence of ASCII characters ending with a carriage return. Each source statement may include up to four fields: a label, an operation (instruction mnemonic or assembler directive), an operand, and a comment.
Labels mark places in a program which other instructions and directives reference .
The operation code field / Instructions Field of an assembly language statement identifies the statement as a machine instruction, an assembler directive, or a macro defined by the programmer:
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A machine instruction is indicated by an instruction mnemonic. An assembly language statement that contains an instruction mnemonic is intended to produce a single executable machine instruction. The operation and use of each instruction is described in the manufacturer's user manual.
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An assembler directive (or pseudo-op) performs some function during the assembly process. It doesn't produce any executable code, but it may assign space for data in the program.
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Macros are defined with the .macro directive
check this out https://www.slideshare.net/dattatraygandhmal/assemblers-33585130
The operand field's interpretation is dependent on the contents of the operation field The operand field, if required, must follow the operation field, and must be preceded bat least one white-space character. The operand field may contain a symbol, an expressionor a combination of symbols and expressions separated by commas.
Register is a temporary storage or working location built into the CPU itself (separate from memory). they are processor components that hold data and address.
The 16 integer registers are 64 bits wide and are called: click here for more informations.
R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
RAX RCX RDX RBX RSP RBP RSI RDI
These are the basic forms of addressing:
- [ number ]
- [ reg ]
- [ reg + reg * scale ] scale is 1, 2, 4, or 8 only
- [ reg + number ]
- [ reg + reg*scale + number ]
The number is called the displacement; the plain register is called the base; the register with the scale is called the index.
A system call allows a userspace program to interface with kernel. The protection mechanism provided by Linux doesn’t allow a userspace programto directly invoke the Kernel. Instead it needs cooperation with some hardware via software interrupts to transition from userspace to kernel space. This prevents userspace program to interfere with the kernel protection mechanisms.
To clearly understand how syscalls works check this out : http://labe.felk.cvut.cz/~stepan/33OSD/files/e0-asm-syscalls.pdf
https://fasterthanli.me/series/reading-files-the-hard-way/part-2
errno is a preprocessor macro used for error indication. It expands to a static (until C++11) thread-local (since C++11) modifiable lvalue of type int.
Several standard library functions indicate errors by writing positive integers to errno. Typically, the value of errno is set to one of the error codes, listed in as macro constants that begin with the letter E, followed by uppercase letters or digits.
The value of errno is 0 at program startup, and although library functions are allowed to write positive integers to errno whether or not an error occurred, library functions never store 0 in errno.
https://en.wikipedia.org/wiki/Errno.h
https://www.tutorialspoint.com/assembly_programming/assembly_system_calls.htm