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