89 lines
2.1 KiB
NASM
89 lines
2.1 KiB
NASM
global _start
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section .data
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str: db '0123456789',0xA
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strSize: equ $ - str
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stdout: equ 1
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section .text
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_start:
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; Print the string
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mov rdx, strSize
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mov rsi, str
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mov rdi, stdout
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mov rax, 1
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syscall
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; mov byte [rsi], 3 == *rsi = 3
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; Move the value of 3 into the address contained in register RSI
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; Note the type specifier
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; byte, word, dword
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; The official syntax is
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;mov byte ptr [rsi], 3
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; But the ptr is implicitly meant with the []'s, so nasm allows
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; us to leave it out
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mov byte [rsi], 'a'
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mov rax, 1
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syscall
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mov word [rsi], 'bc'
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mov rax, 1
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syscall
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mov dword [rsi], 'defg'
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mov rax, 1
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syscall
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; "In 64-bit mode, a memory operand can be referenced by
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; a segment selector and an offset. [...]
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; The offset part of a memory address in 64-bit mode can be
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; specified directly as a static value or through an address
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; computation made up of one or more of the following components:
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; Displacement -- An 8-bit, 16-bit, or 32-bit value.
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; Base -- The value in a 64-bit general-purpose register.
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; Index -- The value in a 64-bit general-purpose register.
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; Scale factor -- A value of 2, 4, or 8 that is multiplied
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; by the index value."
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; In intel syntax, this is written as:
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; [base + scale * index + displacement]
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; Example:
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; [rax + 2 * rbx + 0xdeadbeef]
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; (most of these are optional; see below).
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mov byte [rsi], 'h' ; base only
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mov rax, 1
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syscall
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mov byte [str], 'i' ; displacement only
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mov byte [str+1], 'j' ; displacement only too (assembler evaluates it)
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mov rax, 1
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syscall
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mov rbx, 3
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mov byte [rsi+2], 'k'
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mov byte [rsi+rbx], 'l'
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mov byte [rsi+2*rbx], 'm'
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mov byte [rsi+2*rbx+2], 'n'
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mov rax, 1
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syscall
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; Load Effective Address
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; Calculate the address of the expression, it does NOT load the value!
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mov rbx, 2
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lea rsi, [str+2*rbx] ; = rsi = str + (2 * 2)
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; Due to this powerful feature it is often used with invalid
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; addresses: you can use it to do generic calculations!
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mov rbx, -2
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lea rdx, [strSize+2*rbx] ; print (strSize - 4) characters
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mov rax, 1
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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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