The content of the repository was created as part of the course Programming of microcontrollers and microprocessors at AGH University of Science and Technology. More information can be found here.
To install DDD on your Ubuntu device run setup.sh script, that is located in the main repository folder. During labs we also used as (The portable GNU assembler) and gdb (The GNU Debugger) which are available by default on Ubuntu-type operating systems.
During laboratory classes we worked on Unix OS and used AT&T syntax. It means that constant are prefixed with a "$" and registers are prefixed with a "%". Mnemonics are suffixed with a letter indicating the size of the operands: "q" for qword, "l" for long (dword), "w" for word, and "b" for byte. We write source before the destination, for example mov $5, %eax.
To compile: as -o file_name.o file_name.s
To link: ld -o file_name file_name.o
To run: ./file_name
To compile: as -gstabs -o file_name.o file_name.s
To link: ld -o file_name file_name.o
To run: gdb file_name
More information on debugging using gdb can be found here.
Directives are commands that are part of the assembler syntax but are not related to the x86 processor instruction set. All assembler directives begin with a period (.) More information: oracle, MIT and Texas Instruments Incorporated.
.equ directive is used to give name to some value or symbol. Each time the assembler finds the given names in the program, it will replace the name with the value or a symbol.
.data directive changes the current section to .data. This part of program is used for variable declarations.
.text directive defines the current section as .text. This part of program is used for defining assembly language instructions.
.global directive declares each symbol in the list to be global. A global symbol definition in one file satisfies an undefined reference to the same global symbol in another file. Multiple definitions of a defined global symbol are not allowed. If a defined global symbol has more than one definition, an error occurs. The .globl directive only declares the symbol to be global in scope, it does not define the symbol. .global makes the symbol visible to ld. If you define symbol in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes from a symbol of the same name from another file linked into the same program.
.ascii directive places the characters in string into the object module at the current location but does not terminate the string with a null byte (\0). String must be enclosed in double quotes (")
.asciz is just like .ascii, but each string is followed by a zero byte. The "z" in '.asciz' stands for "zero".
.byte directive generates initialized bytes into the current section. Each byte must be an 8-bit value.
.word directive initializes one or more 16-bit integers
.long directive generates a long integer (32-bit, two's complement value) for each expression into the current section. Each expression must be a 32–bit value and must evaluate to an integer value.
| 64-bit register | Lower 32 bits | Lower 16 bits | Lower 8 bits |
|---|---|---|---|
| rax | eax | ax | al |
| rbx | ebx | bx | bl |
| rcx | ecx | cx | cl |
| rdx | edx | dx | dl |
| rsi | esi | si | sil |
| rdi | edi | di | dil |
| rbp | ebp | bp | bpl |
| rsp | esp | sp | spl |
| r8 | r8d | r8w | r8b |
| r9 | r9d | r9w | r9b |
| r10 | r10d | r10w | r10b |
| r11 | r11d | r11w | r11b |
| r12 | r12d | r12w | r12b |
| r13 | r13d | r13w | r13b |
| r14 | r14d | r14w | r14b |
| r15 | r15d | r15w | r15b |
MOV instruction copies the source operand to the destination operand without affecting the source.
SYSCALL instruction is the fundamental interface between an application and the Linux kernel. The x86_64 architecture introduced a dedicated instruction to make a syscall. It does not access the interrupt descriptor table and is faster. Parameters are passed by setting the general purpose registers as following:
| Syscall # | Param 1 | Param 2 | Param 3 | Param 4 | Param 5 | Param 6 |
|---|---|---|---|---|---|---|
| rax | rdi | rsi | rdx | r10 | r8 | r9 |
NOP instruction performs no operation. It is a one-byte or multi-byte NOP that takes up space in the instruction stream but does not impact machine context, except for the EIP register. Why we use NOPs?
ADD instruction adds the first operand (destination operand) and the second operand (source operand) and stores the result in the destination operand. The destination operand can be a register or a memory location; the source operand can be an immediate, a register, or a memory location. (However, two memory operands cannot be used in one instruction.) When an immediate value is used as an operand, it is sign-extended to the length of the destination operand format.
SUB instruction subtracts the second operand (source operand) from the first operand (destination operand) and stores the result in the destination operand. The destination operand can be a register or a memory location; the source operand can be an immediate, register, or memory location.