path: root/cpu/os.asm

# CONVENTION:
#       return value for functions : in register A
#       arguments for functions : registers A, B, C, D
#       all registers are caller-saved, except SP which is preserved by function calls

.text
    li A msghello
    jal ser_out_msg

    push Z
main_loop:
    # Process serial input
    jal check_input
    jz A end_process_input
    jal run_cmd
end_process_input:

    # Process clock ticking
    pop D
    li B _clock
    lw B 0(B)
    add D D B
    push D

    jz B main_loop
    li A msgtick
    jal ser_out_msg
    j main_loop

# PROCEDURE: run_cmd
# ROLE: execute and clear command stored in cmdline
# ARGUMENTS: none
run_cmd:
    push RA

    li A prompt
    jal ser_out_msg
    li A cmdline
    jal ser_out_msg
    li A endl
    jal ser_out_msg
    
    li A error
    jal ser_out_msg

    li A cmdline_used
    sw Z 0(A)

    pop RA
    jr RA
    

# PROCEDURE: ser_out_msg
# ROLE: write null-terminated string to serial output
# ARGUMENTS: address of string in register A
ser_out_msg:
    li C _output
ser_out_msg_loop:
    lb B 0(A)
    jz B ser_out_msg_ret
    sb B 0(C)
    incri A 1
    j ser_out_msg_loop
ser_out_msg_ret:
    jr RA

# PROCEDURE: check_input
# ROLE: check if an input byte is available. if it is, and is different from '\n' (10), add it to cmdline
# ARGUMENTS: none
# RETURN VALUE: 1 if read byte was '\n', 0 otherwise
# WARNING: no buffer overflow check.
check_input:
    li A _input
    lb A 0(A)
    jz A check_input_ret
    move B A
    sei A A '\n'
    jz A add_b_to_string
    move B Z
add_b_to_string:
    push A
    li A cmdline
    li D cmdline_used
    lw C 0(D)
    add A A C
    sb B 0(A)
    incri C 1
    sw C 0(D)
    pop A
    jz A check_input
check_input_ret:
    jr RA
    

# READ-ONLY PROGRAM DATA
msghello:
    ascii "Hello, world!\n"
msgtick:
    ascii " ..."
prompt:
    ascii "\n$ "
endl:
    ascii "\n"
error:
    ascii "Sorry but I'm to stupid to understand that.\n"


.data
# Space where command-line is buffered from serial input
cmdline:
    byte 256
# Number of bytes used in the command-line buffer
cmdline_used:
    word 1