Computer Architecture and System Programming Laboratory. TA Session 5

Transcription

1 Computer Architecture and System Programming Laboratory TA Session 5

2 Addressing Mode specifies how to calculate effective memory address of an operand x86 64-bit addressing mode rule: up to two of the 64-bit registers and a 64-bit signed constant can be added together to compute a memory address. One of the registers can be optionally pre-multiplied by 2, 4, or 8. Example of right usage mov eax, [rbx] mov [var], ebx mov eax, [rsi-4] mov [rsi+rax], cl mov edx, [rsi+4*rbx] mov dword [myarray + rbx*4 + rax], ecx ; move 4 bytes at the address contained in RBX int EAX ; move the contents of EBX into 4 bytes at address var ; move 4 bytes at address RSI+(-4) into EAX ; move the contents of CL into address RSI+RAX ; move 4 bytes at address RSI+4*EBX into EDX ; move the content of 4 bytes at address myarray + RBX*4 + RAX ; to ECX Examples of wrong usage mov eax, [rbx-rcx] mov [rax+rsi+rdi], ebx ; can only add register values ; at most 2 registers in address computation

3 Addressing Modes - Example #include <stdio.h> #define SIZE 5 extern long dot_product (int * a, int * b, int size); int main () { int a [SIZE] = {1,0,1,0,2}; int b [SIZE] = {1,0,1,0,-2}; long result = dot_product(a,b, SIZE); printf ( %#lx\n ", result); return 0; { MUL / IMUL - multiply unsigned / singed numbers Why are two different instructions needed? Example: MUL: 0xFF x 0xFF = 255 x 255 = (0xFE01) IMUL: 0xFF x 0xFF = ( 1) x ( 1) = 1 (0x0001) global dot_product dot_product: enter sub rsp, 8 ; local variable to hold result mov qword [rsp], 0 mov r8, rdx ; edx would be used for multiplication mov rcx, 0.loop: cmp rcx, r8 je.end mov eax, dword [rdi + (4*rcx)] imul dword [rsi + (4*rcx)] add dword [rsp], eax adc dword [rsp+4], edx inc rcx jmp.loop.end: mov rax, [rsp] ; return value leave ret

4 Linux System Calls System calls are low level functions the operating system makes available to applications via a defined API (Application Programming Interface) System calls represent the interface the kernel presents to user applications. In Linux all low-level I/O is done by reading and writing file handles, regardless of what particular peripheral device is being accessed - a tape, a socket, even your terminal, they are all files. Files are referenced by an integer file descriptor. Low level I/O is performed by making system calls

5 Linux System Call format A system call is explicit request to the kernel, made via a software interrupt put system call number in RAX register set up arguments to the system call first argument goes in RDI, second in RSI, then RDX, R10, R8, and finally R9. If more then 6 arguments needed (not likely), the RDI register contains the memory location where the list of arguments is stored. use instruction result is returned in RAX

6 sys_read read from a file system call number (in RAX): 0 arguments: RDI: file descriptor (to read from it) RSI: pointer to input buffer (to keep a read data into it) RDX: maximal number of bytes to receive (maximal buffer size) return value (in RAX): number of bytes received On errors: 0 (no bits read) or negative number section.bss buffer: resb 1 global _start _start: mov rax, 0 mov rdi, 0 mov rsi, buffer mov rdx, 1 mov rax,60 ; system call number (sys_read) ; file descriptor (stdin) ; buffer to keep the read data ; bytes to read ;exit

7 sys_write write into a file system call number (in RAX): 1 arguments: RDI : file descriptor (to write to it) RSI : pointer to the first byte to write (beginning of the string) RDX : number of bytes (characters) to write return value (in RAX): number of bytes written On errors: negative number section.data msg: db Message',0xa len: equ $ - msg global _start _start: mov rax,1 mov rdi,1 mov rsi, msg mov rdx, len mov rax,60 ;our string, 0xA is newline ;length of our string ;system call number (sys_write) ;file descriptor (stdout) ;message to write ;message length ;exit

8 sys_open - open a file system call number (in RAX): 2 arguments: RDI : pathname of the file to open/create RSI : set file access bits (can be bitwise OR ed together) O_RDONLY (0x000) open for reading only O_WRONLY (0x001) open for writing only O_RDRW (0x002) open for both reading and writing O_APPEND (0x008) open for appending to the end of file O_TRUNC (0x200) truncate to 0 length if file exists O_CREAT (0x100) create the file if it doesn t exist RDX : set file permissions (in a case O_CREAT is set; can be bitwise OR ed together) S_IRWXU 0x700 ; RWX mask for owner S_IRUSR 0x400 ; R(read) for owner USR(user) S_IWUSR 0x200 ; W(write) for owner S_IXUSR 0x100 ; X(execute) for owner return value (in RAX): file descriptor On errors: negative number must choose (at least) one of these section.data filename: db file.txt", 0 handle: dq 0 global _start _start: file_open: mov rax, 2 mov rdi, filename mov rsi, 0x101 mov rdx, S_IRWXU mov [handle], rax mov rax,60 may add some of these ; system call number (sys_open) ; set file name ; set file access bits (O_WRONLY O_CREAT) ; set file permissions ; move file handle to memory ;exit

9 sys_lseek change a file pointer system call number (in RAX): 8 arguments: RDI : file descriptor RSI : offset (number of bytes to move) RDX : where to move from SEEK_SET (0) ; beginning of the file SEEK_CUR (1) ; current position of the file pointer SEEK_END (2) ; end of file return value (in RAX): Current position of the file pointer On errors: SEEK_SET section.data filename: db file.txt", 0 handle: dq 0 global _start _start: file_open: mov rax, 2 ; system call number (sys_open) mov rdi, filename ; set file name mov rsi, 0 ; set file access bits (O_RDONLY ) mov [handle], rax ; move file handle to memory mov rdi, [handle] mov rsi,15 mov rdx,0 mov rax,8 ; file descriptor ; number of byte to move ; move from beginning of the file ; system call number (lseek) mov rax,60 ;exit

10 sys_close close a file system call number (in RAX): 3 arguments: RDI : file descriptor return value (in RAX): nothing meaningful On errors: negative number section.data filename: db file.txt", 0 handle: dq 0 global _start _start: file_open: mov rax, 2 ; system call number (sys_open) mov rdi, filename ; set file name mov rsi, 0 ; set file access bits (O_RDONLY ) mov [handle], rax ; move file handle to memory mov rdi, [handle] mov rax, 3 mov rax,60 ;exit