diff --git a/sys/src/games/nes/cpu.c b/sys/src/games/nes/cpu.c
index 1377cf2fd..c1624c02f 100644
--- a/sys/src/games/nes/cpu.c
+++ b/sys/src/games/nes/cpu.c
@@ -6,6 +6,7 @@
 u16int pc, curpc;
 u8int rA, rX, rY, rS, rP;
 int nmi;
+extern int map;
 
 static u8int
 fetch8(void)
@@ -172,7 +173,13 @@ dec(u16int a)
 static void
 inc(u16int a)
 {
-	memwrite(a, nz(memread(a) + 1));
+	u8int v;
+
+	v = memread(a);
+	memwrite(a, v);
+	v = nz(v + 1);
+	if(!(map == 1 && a >= 0x8000))
+		memwrite(a, v);
 }
 
 static void
@@ -229,6 +236,8 @@ interrupt(int nmi, int brk)
 	rP |= FLAGI;
 }
 
+int trace;
+
 int
 step(void)
 {
@@ -243,6 +252,8 @@ step(void)
 	}
 	curpc = pc;
 	op = fetch8();
+	if(trace)
+		print("%x %x %x %x %x %x %x %x\n", curpc, op, rA, rX, rY, rS, rP, memread(0x2c));
 	switch(op){
 	case 0x00: pc++; interrupt(0, 1); return 7;
 	case 0x01: nz(rA |= indX()); return 6;
@@ -397,12 +408,12 @@ step(void)
 	case 0xC5: cmp(rA, zp()); return 3;
 	case 0xC9: cmp(rA, imm()); return 2;
 	case 0xCD: cmp(rA, abso()); return 4;
+	case 0xD0: if((rP & FLAGZ) == 0) return branch(); pc++; return 3;
 	case 0xD1: cmp(rA, indY(&c)); return 5 + c;
 	case 0xD5: cmp(rA, zpX()); return 4;
 	case 0xD8: rP &= ~FLAGD; return 2;
 	case 0xD9: cmp(rA, absY()); return 4 + ((u8int)a < rY);
 	case 0xDD: cmp(rA, absX()); return 4 + ((u8int)a < rX);
-	case 0xD0: if((rP & FLAGZ) == 0) return branch(); pc++; return 3;
 	case 0xC0: cmp(rY, imm()); return 2;
 	case 0xC4: cmp(rY, zp()); return 3;
 	case 0xC6: dec(fetch8()); return 5;
diff --git a/sys/src/games/nes/mem.c b/sys/src/games/nes/mem.c
index 1354678d8..d018400e8 100644
--- a/sys/src/games/nes/mem.c
+++ b/sys/src/games/nes/mem.c
@@ -193,7 +193,8 @@ memwrite(u16int p, u8int v)
 			p &= 0x2007;
 		switch(p){
 		case PPUCTRL:
-			if((v & PPUNMI) != 0 && (mem[PPUSTATUS] & PPUVBLANK) != 0)
+			if((mem[PPUCTRL] & PPUNMI) == 0 && (v & PPUNMI) != 0 &&
+			   (mem[PPUSTATUS] & PPUVBLANK) != 0)
 				nmi = 1;
 			pput = (pput & 0xF3FF) | ((v & 3) << 10);
 			break;
diff --git a/sys/src/games/nes/nes.c b/sys/src/games/nes/nes.c
index 3b3d62d77..ee72e2b8a 100644
--- a/sys/src/games/nes/nes.c
+++ b/sys/src/games/nes/nes.c
@@ -83,6 +83,8 @@ loadrom(char *file)
 	mapper[map](-1, 0);
 }
 
+extern int trace;
+
 void
 keyproc(void *)
 {
@@ -99,6 +101,8 @@ keyproc(void *)
 		if(buf[0] == 'c'){
 			if(utfrune(buf, Kdel))
 				threadexitsall(nil);
+			if(utfrune(buf, 't'))
+				trace ^= 1;
 		}
 		if(buf[0] != 'k' && buf[0] != 'K')
 			continue;
diff --git a/sys/src/games/nes/ppu.c b/sys/src/games/nes/ppu.c
index cb1e7b7a0..197f70748 100644
--- a/sys/src/games/nes/ppu.c
+++ b/sys/src/games/nes/ppu.c
@@ -5,7 +5,7 @@
 #include "dat.h"
 #include "fns.h"
 
-int ppuy, ppux;
+int ppuy, ppux, odd;
 uchar pic[256*240*4*9];
 extern uchar oam[256];
 
@@ -289,7 +289,11 @@ ppustep(void)
 	if(ppux > 340){
 		ppux = 0;
 		ppuy++;
-		if(ppuy > 261)
+		if(ppuy > 261){
 			ppuy = 0;
+			if(odd && (mem[PPUCTRL] & (BGDISP | SPRITEDISP)) != 0)
+				ppux++;
+			odd ^= 1;
+		}
 	}
 }