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on application entry:
there is a 1MiB - 8KiB area mapped writable and not
executable with guard pages on either side, and rsp is
set to the top of that. all other registers are set to 0.
for all system calls:
rax, rdi, rsi, rdx are in/out paramters.
rbx, rbp, rsp, r12-r15 are preserved.
rcx, rflags, r8-r11 are clobbered.
file result:
0 = success
1 = bad file handle
2 = device error
3 = file system corrupt
4 = tried to read out of bounds
5 = file does not exist
6 = tried to open directory
encode color:
rax in: 0
edi in: r + g * 256 + b * 65536
eax out: encoded color
get framebuffer:
rax in: 1
rax out: pointer to framebuffer
rdi out: width + height * 2 ** 32
esi out: pitch
framebuffer is always 32 bpp. use the encode color syscall
to encode colors. pitch is in dwords, not in bytes.
open file:
rax in: 2
rdi in: pointer to file path
rsi in: file path length
rax out: file result
rdi out: file handle (if rax = success)
get file length:
rax in: 3
rdi in: file handle
rax out: file result
rdi out: file length (if rax = success)
read from file:
rax in: 4
rdi in: pointer to request struct
rax out: file result
request struct:
qword: file handle
qword: start offset in bytes
qword: length in bytes
qword: pointer to buffer
end this process:
rax in: 5
edi in: exit code (signed)
get new pages:
rax in: 6
rdi in: number of pages to allocate
rax out: start of first page
the allocated pages are next to each other, writable, and not executable.
close file:
rax in: 7
rdi in: file handle
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