init.linux


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# Some OS-specific preliminaries for Linux.

# Memory layout
#
#          0 - 0x08047ffff - reserved for the kernel
# 0x08048000 - 0xbffffffff - available for user programs
# 0xc0000000 - 0xfffffffff - reserved for the kernel
== code 0x09000000
== data 0x0a000000

# Syscalls
#
# We don't have libc, so we need to know Linux's precise syscall layout.
== code

# http://man7.org/linux/man-pages/man2/exit.2.html
syscall_exit:  # status/ebx : int
    b8/copy-to-eax 1/imm32
    cd/syscall 0x80/imm8

# http://man7.org/linux/man-pages/man2/read.2.html
syscall_read:  # fd/ebx : int, buf/ecx : address, size/edx : int -> nbytes-or-error/eax : int
    b8/copy-to-eax 3/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/write.2.html
syscall_write:  # fd/ebx : int, buf/ecx : address, size/edx : int -> nbytes-or-error/eax : int
    b8/copy-to-eax 4/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/open.2.html
syscall_open:  # filename/ebx : (address null-terminated-string), flags/ecx : int -> fd-or-error/eax : int
    b8/copy-to-eax 5/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/close.2.html
syscall_close:  # fd/ebx : int -> status/eax
    b8/copy-to-eax 6/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/creat.2.html
syscall_creat:  # filename/ebx : (address null-terminated-string) -> fd-or-error/eax : int
    b8/copy-to-eax 8/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/unlink.2.html
syscall_unlink:  # filename/ebx : (address null-terminated-string) -> status/eax : int
    b8/copy-to-eax 0xa/imm32
    cd/syscall 0x80/imm8
    c3/return

# http://man7.org/linux/man-pages/man2/rename.2.html
syscall_rename:  # source/ebx : (address null-terminated-string), dest/ecx : (address null-terminated-string) -> status/eax : int
    b8/copy-to-eax 0x26/imm32
    cd/syscall 0x80/imm8
    c3/return

# https://github.com/torvalds/linux/blob/fa121bb3fed6313b1f0af23952301e06cf6d32ed/mm/nommu.c#L1352
syscall_mmap:  # arg/ebx : (address mmap_arg_struct) -> status/eax : int
    # the important thing: ebx+4 contains the 32-bit size to be allocated
    b8/copy-to-eax 0x5a/imm32
    cd/syscall 0x80/imm8
    c3/return