RCPU_OS

Kernel implementation running RCPU based on Writing an OS in Rust

Implementation notes

All numbers are unsigned 16 bit (u16)
- Integer overflows can occur
  - For Add, Subtract, Multiply and Divide, the number wraps around (mod 2^16)
  - For Left and Right Shifts, the number gets padded with zeroes
The program expects a 16bit addressable memory space with the program loaded starting at 0
The binary contains pre-allocated space, which will be loaded with the program
All strings are ASCII (7 bit), not Latin-1
RCPU_OS stack grows upwards instead of downwards. RCPU has no way of reading from/writing to SP, so this should not matter to the programs.
RCPU syscalls "return" by pushing the result to the stack
RCPU stream_num is 0 for stdin, 1 for stdout
- Reading from or writing to an invalid stream (everything except 0 and 1 respectively) causes a panic
If Getc cannot read a character, it returns u16::MAX (aka -1 wrapped)
Fgets reads blocking until a null-byte is found or num_characters are read
- If no nullbyte is found, one is put at str_ptr + num_read, so the target string needs to be at least size+1 big if size characters are read
- To enter a nullbyte the F1 key can be pressed