accelerated naked ISR, still has bugs and possible performance improvements

gg.c

@@ -2,8 +2,10 @@
 #include <util/delay.h>
 #include <avr/interrupt.h>
 
+#include "ringbuf_small.c"
 #include "mmc.h"
 
+#include "gg.h"
 
 void timer_init_tiny26(void)
 {
@@ -38,21 +40,20 @@ uint8_t buffer[BUFFER_SIZE];/* buffer for mmc data */
 uint8_t pos = 0;/* current playing position */
 uint8_t refresh_buffer = 0;/* position to start buffer refreshing */
 
-ISR(TIMER1_OVF1_vect)
+ISR(TIMER1_OVF1_vect, ISR_NAKED)
 {
-	//if(!(TIFR & (1 << TOV1))) {continue;}/* interrupt flag polling */
-	//TIFR |= (1 << TOV1);/* done by hardware */
-	
-	overflows--;
-	if(!overflows)
-	{
-		overflows = 1 << (SAMPLE_BITS - 8);
-		pos = (pos + 1) % BUFFER_SIZE;
-	}
-	
-	/* TODO: a-law decoding */
-	/* TODO: 11bit PWM emulation */
-	OCR1A = buffer[pos];/* play sample */
+    __asm__("in r2, 0x3f"); // save sreg
+	if(--cnt_to_ocr_incr){
+        OCR1A = cur_ocr + 1;
+    }
+    if(--cnt_to_next){
+        cur_ocr = next_ocr;
+        cnt_to_ocr_incr = next_cnt_to_incr;
+        cnt_to_next = 8;
+        needs_new_data_flag = 1;
+    }
+    __asm__("out 0x2f,r2"); // restore sreg
+    reti();
 }
 
 

gg.h

@@ -0,0 +1,13 @@
+// contains locked registers
+
+#include <stdint.h>
+
+
+register uint8_t sreg_tmp __asm__("r2");
+register uint8_t cur_ocr __asm__("r3");
+register uint8_t cnt_to_ocr_incr __asm__("r4");
+register uint8_t next_ocr __asm__("r5");
+register uint8_t next_cnt_to_incr __asm__("r6");
+register uint8_t cnt_to_next __asm__("r7");
+register uint8_t needs_new_data_flag __asm__("r8");
+