fix(serial): fixed loop logic in POUTZSTR and some formatting

src/serial.s

@@ -20,13 +20,13 @@
 ; ACCA: Index of the divsor to use in DIVISORS
 ; ACCB: Settings for LCR
 INITUART
+  pshs b        ; Preserve LCR while we set up divisors
   ldx #DIVISORS ; Get DIVISORS base addr
   asla          ; Shift left to *2 the index in order to iter over words.
-  pshs b        ; Save B
   ldd a,x       ; Get divisor into D
   sta UART_DLM  ; Write divisor MSB
   stb UART_DLL  ; Write divisor LSB
-  puls b        ; Restore B
+  puls b
   stb UART_LCR  ; Write LCR
   lda UARTF_FCR_FE|UARTF_FCR_RFR|UARTF_FCR_XFR ; FIFO disable and clear
   sta UART_FCR
@@ -39,10 +39,10 @@ INITUART
 ; ACCA: char to write
 POUTCHAR
   pshs a       ; Preserve char
-NEXTC@
+NOTREADY@
   lda UART_LSR ; Wait until LSR.THRE == 1 then write char
   bita UARTF_LSR_THRE
-  beq NEXTC@
+  beq NOTREADY@
   puls a       ; Restore char
   sta UART_THR ; Write char
   rts
@@ -54,19 +54,19 @@ POUTZSTR
 NEXTC@
   ldb 0,x      ; Get next char from X
   cmpb #$00    ; Make sure that we aren't at a terminator
-  leax 1,x     ; Increment X for next char. we inc here to save bytes if the
-               ; next string is adjacent.
   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
+  leax 1,x     ; Iter to next char. We X++ here because leax modifies CC.Z
   bra NEXTC@   ; Iter to next char
 END@
   puls b,a     ; Restore A and B
   rts
 
+; UART baud rate divisors
 DIVISORS
   fdb $0900 ; 50 baud
   fdb $0600 ; 75 baud