diff --git a/snes/msu1/data.a65 b/snes/msu1/data.a65
index 4aee769..ce9cd8b 100644
--- a/snes/msu1/data.a65
+++ b/snes/msu1/data.a65
@@ -2,11 +2,11 @@
 
 dispcnt		.byt 0
 
-stddur		.byt 0
-altdur		.byt 0
-altcnt		.byt 0
-curdur		.byt 0
-curcnt		.byt 0
+stddur		.byt 0	; standard picture duration in fields
+altdur		.byt 0	; alternate picture duration in fields
+altcnt		.byt 0	; use alternate picture duration every n frames
+curdur		.byt 0	; current valid duration
+curcnt		.byt 0	; current frame count for picture duration
 numframes	.word 0
 firstframe	.byt 0
 charptr		.byt 0
diff --git a/snes/msu1/msu1.a65 b/snes/msu1/msu1.a65
index dbc72f6..55a44c2 100644
--- a/snes/msu1/msu1.a65
+++ b/snes/msu1/msu1.a65
@@ -46,52 +46,52 @@ msu1loop:
 	sta altdur
 	lda $2001
 	sta altcnt
-	stz curcnt
-	ldy numframes
-	dey
+	lda #$01
+	sta curcnt
+	ldx numframes
+	dex
 
 msu1loop2
 	lda isr_flag
 	beq msu1loop2
 	stz isr_flag
-	lda dispcnt
-	cmp #$02
-	beq +
+	lda dispcnt	;load field count
+	cmp #$02	;if >= 2 don't draw anymore
 	bpl +
 	;load half picture
 	lda #$18
 	sta $4301
 	lda #$09
 	sta $4300
-	ldx #16256
-	stx $4305
+	ldy #16256
+	sty $4305
 	lda #$01
 	sta $420b
-+	inc dispcnt
-	lda dispcnt
-	cmp curdur
-	bne msu1loop2
++	inc dispcnt	;inc field count
+	lda dispcnt	;and compare with current duration
+	cmp curdur	;if not reached...
+	bne msu1loop2   ;...wait another field
 
-	lda firstframe
+	lda firstframe	;first frame ready for display?
 	beq +
 
-	lda #$01
+	lda #$01	;then start audio
 	sta MSU_CONTROL
 	stz firstframe
 
 +
-	inc curcnt
-	lda curcnt
-	cmp altcnt
-	bne +
-	stz curcnt
-	lda altdur
+	lda curcnt	;
+	cmp altcnt	;compare with alternation frequency
+	bne +		;if reached...
+	stz curcnt	;...reset current frame count
+	lda altdur	;use alternate duration for next frame
 	bra skip
-+	lda stddur
-skip	sta curdur
-	stz dispcnt
-	dey
-	beq msu1stop
++	lda stddur	;else use normal duration
+	inc curcnt	;and inc current frame count
+skip	sta curdur	;store in current duration
+	stz dispcnt	;reset field counter
+	dex		;countdown total frames
+	beq msu1stop	;stop if end of movie
 
 	;load palette
 	stz $2121
@@ -99,8 +99,8 @@ skip	sta curdur
 	sta $4301
 	lda #$08
 	sta $4300
-	ldx #512
-	stx $4305
+	ldy #512
+	sty $4305
 	lda #$01
 	sta $420b
 	lda charptr
@@ -109,14 +109,14 @@ ptr1
 	lda #$04
 	sta $210b
 	sta charptr
-	ldx #$0000
-	stx $2116
+	ldy #$0000
+	sty $2116
 	jmp msu1loop2
 ptr2
 	stz $210b
 	stz charptr
-	ldx #$4000
-	stx $2116
+	ldy #$4000
+	sty $2116
 	jmp msu1loop2
 
 msu1stop:
diff --git a/snes/msu1/msu1conv.c b/snes/msu1/msu1conv.c
index ad2ca6f..bf88a61 100644
--- a/snes/msu1/msu1conv.c
+++ b/snes/msu1/msu1conv.c
@@ -76,7 +76,7 @@ int main(int argc, char **argv) {
   char inpal[768], outpal[512];
 
   FILE *in, *out;
-  int fileno=1;
+  int fileno=0, startframe=0;
   uint16_t numcolors;
   char filename[80];
   out=fopen("out.msu", "wb");
@@ -87,7 +87,14 @@ int main(int argc, char **argv) {
   fwrite(&filename, 5, 1, out); /* write padding for now */
   while(1) {
     sprintf(filename, "%08d.tga", fileno++);
-    if((in=fopen(filename, "rb"))==NULL) break;
+    if((in=fopen(filename, "rb"))==NULL) {
+      if(fileno==1) {
+        startframe = 1;
+        continue;
+      } else {
+        break;
+      }
+    }
     fseek(in, 5, SEEK_SET);
     fread(&numcolors, 2, 1, in);
     fseek(in, 18, SEEK_SET);
@@ -104,6 +111,7 @@ int main(int argc, char **argv) {
     convertpalette(inpal, outpal);
     fwrite(outpal, 512, 1, out);
   }
+  fileno-=startframe;
   numframes_l=fileno&0xff;
   numframes_h=fileno>>8;
   fseek(out, 0, SEEK_SET);
diff --git a/snes/msu1/spc.a65 b/snes/msu1/spc.a65
index 4526b5a..b2d44f9 100644
--- a/snes/msu1/spc.a65
+++ b/snes/msu1/spc.a65
@@ -6,10 +6,13 @@ spc_upload:
 -	cmp $2140
 	bne -
 
-	; IPL protocol: $2142-3 = target address
+	; IPL portmap outside transfer state:
+	; $2141   = command (transfer / run)
+	; $2142-3 = target address
+	; $2140   = trigger
 	lda @spcaddr
 	sta $2142
-	ldx #$01
+	ldx #$01	; transfer
 	stx $2141
 	ldx #$cc
 	stx $2140
@@ -17,6 +20,9 @@ spc_upload:
 -	cpx $2140
 	bne -
 
+	; IPL portmap inside transfer state:
+	; $2140 = sequence number
+	; $2141 = payload
 	; init counters
 	sep #$20 : .as
 	ldx #$00	; sequence counter
@@ -35,8 +41,8 @@ spc_loop
 spc_end
 	rep #$20 : .al
 	lda @spcexec
-	sta $2142
-	stz $2141	; data = 0
+	sta $2142	; set exec address
+	stz $2141	; command: run
 	ldx $2140
 	inx
 	inx