use names instead of registers

main.S

@@ -7,13 +7,36 @@
 #define SP r1
 #define SR r2
 
+;; --- register usage --------------------------------
+;; r4: synchronization between main loop and isr
+;;     bit0 signals byte done from isr
+;; r5: data byte to be handled by isr
+;; r6: bit-counter, only used within isr
+;; r7: screen data start/next pointer
+;; r8: screen data end pointer
+;; r9: next screen data byte, prepared in mainloop
+
+#define SIGNAL_INIT_VALUE 0x00
+#define SIGNAL_BYTE_DONE 0x01
+#define SIGNAL_ISR_ENABLE 0x02
+#define SIGNAL_REGISTER r4
+
+#define DATA_BYTE_REGISTER r5
+#define NEXT_DATA_BYTE_REGISTER r9
+#define DATA_NEXT_ADDRESS_REGISTER r7
+#define DATA_LAST_ADDRESS_REGISTER r8
+
+#define BIT_COUNTER_REGISTER r6
+#define BIT_COUNTER_INIT_VALUE 0x01
+;; ---------------------------------------------------
+
 
   .section ".rodata","a"
 screendata:
-  .byte   0x00
+  .byte   0b00000001
-  .byte   0x01
+  .byte   0b00000011
-  .byte   0x02
+  .byte   0b00000111
-  .byte   0x03
+  .byte   0b00001111
 screendataend:
   .byte   0xff
 red:
@@ -43,133 +66,121 @@ green:
   .section ".text","ax",@progbits
 _start:
   ;; disable watchdog
-  mov.w   #WDTPW|WDTHOLD,&WDTCTL
+  mov.w   #WDTPW|WDTHOLD, &WDTCTL
 
   ;; configure clock system to the highest frequency
-  mov.b   #DCO0|DCO1|DCO2,&DCOCTL
+  mov.b   #DCO0|DCO1|DCO2, &DCOCTL
-  mov.b   #XT2OFF|RSEL0|RSEL1|RSEL2|RSEL3,&BCSCTL1
+  mov.b   #XT2OFF|RSEL0|RSEL1|RSEL2|RSEL3, &BCSCTL1
-  mov.b   #0,&BCSCTL2
+  mov.b   #0, &BCSCTL2
-  mov.b   #0,&BCSCTL3
+  mov.b   #0, &BCSCTL3
 
   ;; initialize stack pointer with value from linker
   mov.w   #__stack, SP
 
-  mov.w   #0xaaaa, r7
-  mov.w   #0x5555, r8
-
 init:
   ;; configuration of GPIO Ports
-  mov.b   #BIT0|BIT1|BIT2,&P1DIR
+  mov.b   #BIT0|BIT1|BIT2, &P1DIR
-  mov.b   #BIT1|BIT4,&P2DIR
+  mov.b   #BIT1|BIT4, &P2DIR
-  mov.b   #BIT1|BIT4,&P2SEL
+  mov.b   #BIT1|BIT4, &P2SEL
 
   ;; timer configuration
   ;;  configure and stop timer
   ;;  cycle time is 56.25ns
-  mov.w   #ID_0|MC_0|TACLR|TASSEL_2,&TA1CTL
+  mov.w   #ID_0|MC_0|TACLR|TASSEL_2, &TA1CTL
   ;;  2.0us 
-  mov.w   #45,&TA1CCR0
+  mov.w   #45, &TA1CCR0
   ;;  a bit less
-  mov.w   #10,&TA1CCR1
+  mov.w   #10, &TA1CCR1
-  mov.w   #22,&TA1CCR2
+  mov.w   #22, &TA1CCR2
   ;;  configure output mode for TA0.1
-  mov.w   #CCIE,&TA1CCTL0
+  mov.w   #CCIE, &TA1CCTL0
-  mov.w   #OUTMOD_7,&TA1CCTL1
+  mov.w   #OUTMOD_7, &TA1CCTL1
-  mov.w   #OUTMOD_7,&TA1CCTL2
+  mov.w   #OUTMOD_7, &TA1CCTL2
 
   ;; initialize bit-counter for isr
-  mov.b   #0x01,r6
+  mov.b   #BIT_COUNTER_INIT_VALUE, BIT_COUNTER_REGISTER
   ;; initialize isr-sync register
-  mov.b   #0x00,r4
+  mov.b   #SIGNAL_INIT_VALUE, SIGNAL_REGISTER
 
-  ;; screen data start/next into r7
+  ;; screen data start/next into DATA_NEXT_ADDRESS_REGISTER
-  mov.w   #screendata, r7
+  mov.w   #screendata, DATA_NEXT_ADDRESS_REGISTER
-  ;; screen data end into r8
+  ;; screen data end into DATA_LAST_ADDRESS_REGISTER
-  mov.w   #screendataend, r8
+  mov.w   #screendataend, DATA_LAST_ADDRESS_REGISTER
-  ;; load first screen data value into r5
+  ;; load first screen data value into DATA_BYTE_REGISTER
-  mov.b   @r7,r5
+  mov.b   @DATA_NEXT_ADDRESS_REGISTER, DATA_BYTE_REGISTER
-  inc.w   r7
+  inc.w   DATA_NEXT_ADDRESS_REGISTER
 
   ;; start timer in up mode
-  bis.w   #MC0,&TA1CTL
+  bis.w   #MC0, &TA1CTL
   ;; enable interrupts
   eint
 
 
-
-
-  ;; r4: synchronization between mainloop and isr
-  ;; r5: data byte to be handled by isr
 mainloop:
   ;; prepare next byte to handle by isr
-  cmp.w   r7,r8
+  ;; first, check whether we are already at the end of screen data
+  cmp.w   DATA_NEXT_ADDRESS_REGISTER, DATA_LAST_ADDRESS_REGISTER
   jnz     mainloop_prepare_next_byte
-  mov.w   &screendata,r7
+  ;; in that case rollover to the start again
+  mov.w   #screendata, DATA_NEXT_ADDRESS_REGISTER
 mainloop_prepare_next_byte:
-  mov.b   @r7,r9
+  ;; load the byte from the screen data next pointer
-  inc.w   r7
+  mov.b   @DATA_NEXT_ADDRESS_REGISTER, NEXT_DATA_BYTE_REGISTER
+  ;; and increase the pointer
+  inc.w   DATA_NEXT_ADDRESS_REGISTER
 
 mainloop_wait_for_isr:
-  ;; check bit0 in sync register
+  ;; check bit0 in sync register, wait for the signal from 
-  bit     #0x01,r4
+  ;; the isr
+  bit     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER
   jz      mainloop_wait_for_isr
 
   ;; load data
-  mov.b   r9,r5
+  mov.b   NEXT_DATA_BYTE_REGISTER, DATA_BYTE_REGISTER
-  mov.b   #0x00,r4
+  ;; clear the signal
-  ;; signal reload
+  bic     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER
-  bis     #BIT2,&P1OUT
+  ;; mark the cycle
-  bic     #BIT2,&P1OUT
+  bis     #BIT2, &P1OUT
+  bic     #BIT2, &P1OUT
 
   ;; continue
   jmp     mainloop
 
 
-
-
 ; --- timer isr ---
   ;; r6: exclusively used by isr as bit-counter
 timer1_a0_isr:
   ;; func begin marker
-  bis     #BIT0,&P1OUT
+  bis     #BIT0, &P1OUT
   ;; check isr idle bit
-  bit     #BIT1,r4
+  bit     #SIGNAL_ISR_ENABLE, SIGNAL_REGISTER
   jnz     timer1_a0_isr_exit
 
   ;; shift msb of data register r5 into carry flag and set or reset P1.1 accordingly
-  rla.b   r5
+  rla.b   DATA_BYTE_REGISTER
   jnc     timer1_a0_isr_false_bit
-  bis     #BIT1,&P1OUT
+  bis     #BIT1, &P1OUT
   jmp     timer1_a0_isr_end
 timer1_a0_isr_false_bit:
-  bic     #BIT1,&P1OUT
+  bic     #BIT1, &P1OUT
    
   ;; shift bit-counter, after eight shifts signal byte done and reset bit-counter
 timer1_a0_isr_end:
-  rla.b   r6
+  rla.b   BIT_COUNTER_REGISTER
   jnc     timer1_a0_isr_exit
 
   ;; reset bit-counter
-  mov.b   #0x01,r6
+  mov.b   #BIT_COUNTER_INIT_VALUE, BIT_COUNTER_REGISTER
   ;; signal byte done
-  mov.b   #0x01,r4
+  bis     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER
 
 timer1_a0_isr_exit:
   ;; func end marker
-  bic     #BIT0,&P1OUT
+  bic     #BIT0, &P1OUT
   reti
 
 
 
-;timer0_a1_isr:
-;  reti
-
-; --- interrupt vectors ---
-;  .section "__interrupt_vector_9","ax",@progbits
-;  .word   timer0_a1_isr
-  .section "__interrupt_vector_14","ax",@progbits
-  .word   timer1_a0_isr
-
   ;; .resetvec comes from linker
   .section ".resetvec","ax",@progbits
   .word   _start