feat(serial): added input char routine and some small fixes

src/serial.s

@@ -30,41 +30,11 @@ INITUART
   stb UART_LCR  ; Write LCR
   lda #(UARTF_FCR_FE|UARTF_FCR_RFR|UARTF_FCR_XFR) ; FIFO disable and clear
   sta UART_FCR
-  lda #0
+  clra
   sta UART_IER  ; Polled mode
   sta UART_MCR  ; Reset DTR, RTS
   rts
 
-; Prints a character in polled non-FIFO mode (INITUART state).
-; ACCA: char to write
-POUTCHAR
-  pshs a       ; Preserve char
-NOTREADY@
-  lda UART_LSR ; Wait until LSR.THRE == 1 then write char
-  bita UARTF_LSR_THRE
-  beq NOTREADY@
-  puls a       ; Restore char
-  sta UART_THR ; Write char
-  rts
-
-; Prints a null terminated string in polled non-FIFO mode (INITUART state).
-; X: start of zstring
-POUTZSTR
-  pshs a,b     ; Preserve A and B
-NEXTC@
-  ldb ,x+      ; Get next char from X and increment X for next time
-  cmpb #$00    ; Make sure that we aren't at a terminator
-  beq END@
-NOTREADY@
-  lda UART_LSR ; Wait until LSR.THRE == 1 then write char
-  bita UARTF_LSR_THRE
-  beq NOTREADY@
-  stb UART_THR ; Write char
-  bra NEXTC@   ; Iter to next char
-END@
-  puls b,a     ; Restore A and B
-  rts
-
 ; UART baud rate divisors
 DIVISORS
   fdb $0900 ; 50 baud
@@ -82,3 +52,47 @@ DIVISORS
   fdb $0010 ; 7200 baud
   fdb $000C ; 9600 baud
   fdb $0006 ; 19200 baud
+
+; Prints a character in polled non-FIFO mode (INITUART state).
+; ACCA: char to write
+POUTCHAR
+  pshs a       ; Preserve char
+NOTREADY@
+  lda UART_LSR ; Wait until LSR.THRE == 1 then write char
+  bita #UARTF_LSR_THRE
+  beq NOTREADY@
+  puls a       ; Restore char
+  sta UART_THR ; Write char
+  rts
+
+; Prints a null terminated string in polled non-FIFO mode (INITUART state).
+; X: start of zstring
+POUTZSTR
+  pshs a,b     ; Preserve A and B
+NEXTC@
+  ldb ,x+      ; Get next char from X and increment X for next time
+  cmpb #$00    ; Make sure that we aren't at a terminator
+  beq END@
+NOTREADY@
+  lda UART_LSR ; Wait until LSR.THRE == 1 then write char
+  bita #UARTF_LSR_THRE
+  beq NOTREADY@
+  stb UART_THR ; Write char
+  bra NEXTC@   ; Iter to next char
+END@
+  puls b,a     ; Restore A and B
+  rts
+
+; Reads a char in polled mode non-FIFO mode (INITUART state).
+; A: Filled with char from RX buffer or NUL ($00) if no char is ready
+PINCHAR
+  lda UART_LSR ; See if a char is ready
+  ; Mask of possible UART error codes | data ready flag ($9F)
+  anda #(UARTF_LSR_DR|UARTF_LSR_OE|UARTF_LSR_PE|UARTF_LSR_FE|UARTF_LSR_BI|UARTF_LSR_FIFO)
+  bita #UARTF_LSR_DR
+  beq NOCHAR@
+  lda UART_RBR ; Pull char from RX buffer
+  rts
+NOCHAR@
+  clra         ; Fill A with '\0'.
+  rts