164 lines
2.5 KiB
NASM
164 lines
2.5 KiB
NASM
global _start
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section .bss
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BUF_SIZE: equ 64
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buf1: resb 64
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buf2: resb 64
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section .text
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_start:
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; Calling convention of a non leaf function is the same
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; as a leaf function
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mov rdi, 40579
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mov rsi, buf1
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mov rdx, BUF_SIZE
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call uint2str
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; write result to stdout
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mov rdx, rax
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mov rax, 1
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mov rdi, 1
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mov rsi, buf1
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syscall
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; Calculate !5
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mov rdi, 5
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call factorial
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; Convert result to string
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mov rdi, rax
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mov rsi, buf2
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mov rdx, BUF_SIZE
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call uint2str
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; Write !5 to stdout
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mov rdx, rax
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mov rax, 1
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mov rdi, 1
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mov rsi, buf2
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syscall
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; Exit
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mov rax, 60
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xor rdi, rdi
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syscall
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; size_t uint2str(unsigned number, char * buffer, size_t bufferSize)
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; Returns the number of bytes written to the buffer
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uint2str:
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push rbp ; Store the rbp register on the stack
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mov rbp, rsp ; Overwrite rbp with the function local stack pointer
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; Backup the remaining callee saved registers
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push rbx
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push r12
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push r13
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push r14
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push r15
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; Overwrite the saved registers for demonstration purposes
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mov r12, rdi ; number
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mov r13, rsi ; buffer
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mov r14, rdx ; bufferSize
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xor r15, r15 ; counter
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mov rax, r12 ; working copy of number
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.loop:
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cmp rax, 0
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je .done ; n == 0
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cmp r15, r14
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jge .done ; i > BUF_SIZE
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; n % 10
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xor rdx, rdx
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mov rdi, 10
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div rdi ; rax = rax / rdi
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add dl, '0' ; rdx = rax % rdi
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mov byte [r13 + r15], dl
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inc r15
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jmp .loop
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.done:
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mov byte [r13 + r15], 0x0a ; append newline
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inc r15
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; Call reverse string routine to fix the order
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mov rdi, r13 ; buffer
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lea rsi, [r15 - 1] ; number of characters written, excluding newline
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call revstr
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mov rax, r15
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; Restore saved registers
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pop r15
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pop r14
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pop r13
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pop r12
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pop rbx
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; restore rsp from rbp
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mov rsp, rbp
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pop rbp ; restore rbp
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ret
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; unsigned factorial(unsigned n)
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; recursive!
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factorial:
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; save registers
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push rbp
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mov rbp, rsp
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push rbx
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push r12
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push r13
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push r14
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push r15
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mov r12, rdi; n
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cmp r12, 0
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je .zero
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mov rbx, rdi
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dec rdi
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call factorial ; recursion!
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xor rdx, rdx
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mul rbx
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jmp .done
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.zero:
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mov rax, 1 ; !0 = 1
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.done:
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; Restore saved registers
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pop r15
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pop r14
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pop r13
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pop r12
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pop rbx
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; restore rsp from rbp
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mov rsp, rbp
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pop rbp ; restore rbp
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ret
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; void revstr(char * s, size_t size)
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revstr:
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xor r8, r8 ; lowerbound
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lea r9, [rsi - 1] ; upperbound
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.loop:
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cmp r8, r9
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jge .done
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mov cl, byte [rdi + r8] ; tmp = s[lowerbound]
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xchg cl, byte [rdi + r9] ; tmp <-> s[upperbound]
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mov byte [rdi + r8], cl ; s[lowerbound] = tmp
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inc r8
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dec r9
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jmp .loop
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.done:
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ret
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