Started working on characters

pat80-io-devices/composite-pal-adapter/software/avr-assembly/communication.asm

@@ -8,11 +8,17 @@
 ; The data port is PORTB. The CLK (clock) signal is on PORTD0
 ; and the RS (register select) on PORTD1
 
+; INTERNAL POINTER:
+; Internally, the last screen position is represented by 24 bits in two registers:
+; POS_COARSE: Register Z (16-bit, r31 and r30): Coarse position. Points to one of the chunks
+;    (character columns). 52 chunks per row, 304 rows. Used for character position, as
+;    1 chunk = 1 byte = 1 character.
+; POS_FINE: Register r24: Fine position. Represents the bit inside the chunk selected by POS_COARSE.
+;    Ignored in character mode (the character is always aligned to column.)
+
 ; Initializes and waits for a byte on PORTB
 comm_init:
-    ; Set Z to framebuffer start
-	ldi r31, high(FRAMEBUFFER<<1)
-	ldi r30, low(FRAMEBUFFER<<1)
+	call cursor_pos_home ; Set cursor to 0,0
     comm_wait_byte:
         in r24, PINB   ; read PORTB
         ; Check continuously CLK until a LOW is found
@@ -27,3 +33,10 @@ comm_init:
 		brne comm_wait_byte	; if not 0, repeat h_picture_loop
         jmp comm_init ; filled all memory: reset framebuffer position
 
+; Sets the cursor to 0,0 and clears fine position
+cursor_pos_home:
+	; Set Z to framebuffer start
+	ldi POS_COARSE_H, high(FRAMEBUFFER<<1)
+	ldi POS_COARSE_L, low(FRAMEBUFFER<<1)
+	clr POS_FINE
+	ret