1 files changed, 98 insertions, 5 deletions
diff --git a/cpu/os.asm b/cpu/os.asm
index 5d58ff7..4a7449a 100644
--- a/cpu/os.asm
+++ b/cpu/os.asm
@@ -1,15 +1,58 @@
+# 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
-init:
+    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)
-    jz B init
     add D D B
     push D
+
+    jz B main_loop
     li A msgtick
     jal ser_out_msg
-    pop D
-    j init
+    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:
@@ -21,5 +64,55 @@ 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 "Tick!"
+    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
+
+
+
+
+