Benji Dial
63167f223e
currently, BAR fields of IDE drives are all returning zero, and the ATA read function isn't working. i'm not sure why. i'm going to work on VESA next, and come back to the IDE driver later
2.2 KiB
Assembly Programming on Portland
Assembly programming on Portland OS works pretty similarly to other OS's, but the linking phase varies in order to get the proper Portland Executable format for the output.
Portland-specific quirks
The linker script used in the "Compiling and linking" section only recognizes .text, .rodata, .data and .bss sections, so please only use these. All of these except .bss are treated the same when the executable is loaded, so it doesn't really matter which section you put anything in except for alignment considerations, but it is still helpful for debugging and for readability to put things into the proper sections. This linker script provides a symbol __pl_length at the end of the .data section, used for generating the PLE header, so don't name any symbols this.
Portland ABI
At program startup¸ we are in 32-bit protected mode in ring 3. The data and stack segments are equal, and these and the code segment refer to a region of memory containing .text, .rodata and .data (all three loaded verbatim from the executable) at address zero, followed by an uninitialized region of memory the size of .bss. The stack pointer is initialized to the end of this region, and the instruction pointer is initialized to the _entry label.
See ints.txt for information about system calls.
Compiling and linking
The following commands are used to compile and link an (NASM) assembly program into a Portland Executable. Replace the parts in all caps (except the -O argument to objcopy) with the relevant file names. The user_elf.ld file can be found at src/user-elf.ld in the kernel repository.
nasm -f elf32 SOURCE_FILE_1.asm -o SOURCE_FILE_1.o
nasm -f elf32 SOURCE_FILE_2.asm -o SOURCE_FILE_2.o
...
ld -T user_elf.ld SOURCE_FILE_1.o SOURCE_FILE_2.o ... -o PROGRAM.elf
objcopy -O binary PROGRAM.elf PROGRAM.ple
After running each of these commands, PROGRAM.elf is a regular 32-bit ELF file containing the compiled program, with symbol table and all, useful for debugging. The .ple_head section contains the PLE header placed at the start of the executable by objcopy, and can be ignored when examining the ELF file. The PROGRAM.ple file is an executable file that can be run by Portland OS.