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# 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
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
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