some comments

main.S

@@ -54,9 +54,6 @@ _start:
   ;; 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
@@ -77,6 +74,14 @@ init:
   mov.w   #OUTMOD_7,&TA1CCTL1
   mov.w   #OUTMOD_7,&TA1CCTL2
 
+  ;; 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
+
   ;; initialize bit-counter for isr
   mov.b   #0x01,r6
   ;; initialize isr-sync register
@@ -96,28 +101,30 @@ init:
   eint
 
 
-
-
-  ;; r4: synchronization between mainloop and isr
-  ;; r5: data byte to be handled by isr
 mainloop:
   ;; prepare next byte to handle by isr
+  ;; first, check whether we are already at the end of screen data
   cmp.w   r7,r8
   jnz     mainloop_prepare_next_byte
+  ;; in that case rollover to the start again
   mov.w   #screendata,r7
 mainloop_prepare_next_byte:
+  ;; load the byte from the screen data next pointer
   mov.b   @r7,r9
+  ;; and increase the pointer
   inc.w   r7
 
 mainloop_wait_for_isr:
-  ;; check bit0 in sync register
+  ;; check bit0 in sync register, wait for the signal from 
+  ;; the isr
   bit     #0x01,r4
   jz      mainloop_wait_for_isr
 
   ;; load data
   mov.b   r9,r5
+  ;; clear the signal
   mov.b   #0x00,r4
-  ;; signal reload
+  ;; mark the cycle
   bis     #BIT2,&P1OUT
   bic     #BIT2,&P1OUT
 
@@ -125,8 +132,6 @@ mainloop_wait_for_isr:
   jmp     mainloop
 
 
-
-
 ; --- timer isr ---
   ;; r6: exclusively used by isr as bit-counter
 timer1_a0_isr: