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In order to create user mode processes (without implementing a program
loader right now) I compile simple C programs (with a main() function)
to object files and link them with a ld linker script with
OUTPUT_FORMAT set to binary, creating a .com
file. (I use this ending in memory of simple DOS programs and because
yesterday the original IBM PC turned 30 years.)
Then comes some Unix
shell script magic which pipes the binary through hexdump, fmt, and sed
in order to create a char array declaration like this:
static char procbin[] __attribute__ ((aligned (4096))) = {
0x83, 0xec, 0x10, 0xc7, 0x44, 0x24, 0x0c, 0x00, 0x00, 0x00,
0x00, 0xc7, 0x44, 0x24, 0x08, 0x01, 0x00, 0x00, 0x00, 0xeb,
0x10, 0x8b, 0x44, 0x24, 0x08, 0x01, 0x44, 0x24, 0x0c, 0xff,
0x44, 0x24, 0x0c, 0xff, 0x44, 0x24, 0x08, 0x83, 0x7c, 0x24,
0x08, 0x09, 0x7e, 0xe9, 0xb8, 0xcd, 0xab, 0x00, 0x00, 0xcd,
0x80, 0x8b, 0x44, 0x24, 0x0c, 0x83, 0xc4, 0x10, 0xc3, 0x00
};
That can be pasted into the Ulix source code and will create a page-aligned
block holding the program. This is then mapped to page 0, and user mode is
entered with an IRET to address 0: The program starts.
In order to check what code has been produced by the compiler, I use
the disassembler udis86.
Resources used:
udis86 disassembler library with udcli client,
http://udis86.sourceforge.net/
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