diff --git a/Makefile b/Makefile
index f3ba17a..b909e00 100644
--- a/Makefile
+++ b/Makefile
@@ -256,12 +256,15 @@ ifdef NO_FILE_CHOOSER
 CHOOSER:=nuklear_ui/filechooser_nulll.o
 endif
 
+#The generated Z80 core is normally selected together with the generated 68K
+#core, but the two are independent. NEW_Z80 can be set on its own to pair the
+#JIT 68K with the generated Z80, which is useful for narrowing down which core
+#a timing difference comes from.
 ifdef NEW_CORE
-Z80OBJS=z80.o z80inst.o
+NEW_Z80:=1
 M68KOBJS+= m68k.o
 CFLAGS+= -DNEW_CORE
 else
-Z80OBJS=z80inst.o z80_to_x86.o
 ifeq ($(CPU),x86_64)
 M68KOBJS+= m68k_core.o m68k_core_x86.o
 TRANSOBJS+= gen_x86.o backend_x86.o
@@ -272,6 +275,13 @@ TRANSOBJS+= gen_x86.o backend_x86.o
 endif
 endif
 endif
+
+ifdef NEW_Z80
+Z80OBJS=z80.o z80inst.o
+CFLAGS+= -DNEW_Z80
+else
+Z80OBJS=z80inst.o z80_to_x86.o
+endif
 AUDIOOBJS=ym2612.o ymf262.o ym_common.o psg.o wave.o flac.o vgm.o event_log.o render_audio.o rf5c164.o
 CONFIGOBJS=config.o tern.o util.o paths.o
 NUKLEAROBJS=$(FONT) $(CHOOSER) nuklear_ui/blastem_nuklear.o nuklear_ui/sfnt.o nuklear_ui/debug_ui.o
diff --git a/coleco.c b/coleco.c
index 66d854e..afaba30 100644
--- a/coleco.c
+++ b/coleco.c
@@ -10,7 +10,7 @@
 #include "bindings.h"
 #include "saves.h"
 
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 #define Z80_CYCLE cycles
 #define Z80_OPTS opts
 #define z80_handle_code_write(...)
@@ -42,7 +42,7 @@ static uint8_t coleco_controller_read(uint32_t location, void *vcontext)
 
 static void update_interrupts(coleco_context *coleco)
 {
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	if (coleco->z80->nmi_cycle == CYCLE_NEVER) {
 #else
 	if (coleco->z80->nmi_start == CYCLE_NEVER) {
@@ -203,7 +203,7 @@ static uint8_t load_state(system_header *system, uint8_t slot)
 	coleco_context *coleco = (coleco_context *)system;
 	char *statepath = get_slot_name(system, slot, "state");
 	uint8_t ret;
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	if (!coleco->z80->native_pc) {
 		ret = get_modification_time(statepath) != 0;
 		if (ret) {
@@ -349,7 +349,7 @@ static void soft_reset(system_header *system)
 {
 	coleco_context *coleco = (coleco_context *)system;
 	z80_assert_reset(coleco->z80, coleco->z80->Z80_CYCLE);
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	coleco->z80->target_cycle = coleco->z80->sync_cycle = coleco->z80->Z80_CYCLE;
 #endif
 }
@@ -373,7 +373,7 @@ static void request_exit(system_header *system)
 {
 	coleco_context *coleco = (coleco_context *)system;
 	coleco->should_return = 1;
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	coleco->z80->target_cycle = coleco->z80->sync_cycle = coleco->z80->Z80_CYCLE;
 #endif
 }
diff --git a/coleco.h b/coleco.h
index d44bbfd..7ce9ac9 100644
--- a/coleco.h
+++ b/coleco.h
@@ -4,7 +4,7 @@
 #include "system.h"
 #include "vdp.h"
 #include "psg.h"
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 #include "z80.h"
 #else
 #include "z80_to_x86.h"
diff --git a/debug.h b/debug.h
index d154728..22db361 100644
--- a/debug.h
+++ b/debug.h
@@ -5,9 +5,12 @@
 #include "tern.h"
 #ifdef NEW_CORE
 #include "m68k.h"
-#include "z80.h"
 #else
 #include "m68k_core.h"
+#endif
+#ifdef NEW_Z80
+#include "z80.h"
+#else
 #include "z80_to_x86.h"
 #endif
 #include "upd78k2.h"
diff --git a/genesis.c b/genesis.c
index aaa40d3..4f41271 100644
--- a/genesis.c
+++ b/genesis.c
@@ -43,15 +43,17 @@ uint32_t MCLKS_PER_68K;
 #define MAX_SOUND_CYCLES 100000
 #endif
 
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 #define Z80_CYCLE cycles
 #define Z80_OPTS opts
 #define z80_handle_code_write(...)
-#define int_num int_priority
 #else
 #define Z80_CYCLE current_cycle
 #define Z80_OPTS options
 #endif
+#ifdef NEW_CORE
+#define int_num int_priority
+#endif
 
 void genesis_serialize(genesis_context *gen, serialize_buffer *buf, uint32_t m68k_pc, uint8_t all)
 {
@@ -451,7 +453,7 @@ static void adjust_int_cycle(m68k_context * context, vdp_context * v_context)
 static void z80_next_int_pulse(z80_context * z_context)
 {
 	genesis_context * gen = z_context->system;
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	z_context->int_cycle = vdp_next_vint_z80(gen->vdp);
 	z_context->int_end_cycle = z_context->int_cycle + Z80_INT_PULSE_MCLKS;
 	z_context->int_value = 0xFF;
@@ -468,7 +470,7 @@ static void sync_z80(genesis_context *gen, uint32_t mclks)
 	z80_context *z_context = gen->z80;
 #ifndef NO_Z80
 	if (z80_enabled) {
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 		if (z_context->int_cycle == 0xFFFFFFFFU) {
 			z80_next_int_pulse(z_context);
 		}
@@ -515,14 +517,31 @@ static void sync_sound(genesis_context * gen, uint32_t target)
 #define REFRESH_INTERVAL 128
 #define REFRESH_DELAY 2
 
-void gen_update_refresh(m68k_context *context)
+//Advances the refresh counter by elapsed master cycles and returns the refresh
+//delay incurred along the way. The delay cycles are themselves time in which the
+//counter keeps running, so they have to be fed back into it. Doing that in a loop
+//rather than in a single division is what makes the result independent of how
+//large the chunks are that the caller happens to hand us; the interpreter and the
+//JIT sync at different granularity, so anything chunk-dependent makes the two
+//cores drift apart.
+static uint32_t refresh_advance(genesis_context *gen, uint32_t elapsed)
 {
 	uint32_t interval = MCLKS_PER_68K * REFRESH_INTERVAL;
+	uint32_t delay = 0;
+	gen->refresh_counter += elapsed;
+	while (gen->refresh_counter >= interval) {
+		gen->refresh_counter -= interval;
+		gen->refresh_counter += REFRESH_DELAY * MCLKS_PER_68K;
+		delay += REFRESH_DELAY * MCLKS_PER_68K;
+	}
+	return delay;
+}
+
+void gen_update_refresh(m68k_context *context)
+{
 	genesis_context *gen = context->system;
-	gen->refresh_counter += context->cycles - gen->last_sync_cycle;
+	context->cycles += refresh_advance(gen, context->cycles - gen->last_sync_cycle);
 	gen->last_sync_cycle = context->cycles;
-	context->cycles += REFRESH_DELAY * MCLKS_PER_68K * (gen->refresh_counter / interval);
-	gen->refresh_counter = gen->refresh_counter % interval;
 }
 
 void gen_update_refresh_free_access(m68k_context *context)
@@ -532,21 +551,12 @@ void gen_update_refresh_free_access(m68k_context *context)
 	if (before < gen->last_sync_cycle) {
 		return;
 	}
-	//Add refresh delays for any accesses that happened beofre the current one
-	gen->refresh_counter += before - gen->last_sync_cycle;
-	uint32_t interval = MCLKS_PER_68K * REFRESH_INTERVAL;
-	uint32_t delay = REFRESH_DELAY * MCLKS_PER_68K * (gen->refresh_counter / interval);
-	if (delay) {
-		//To avoid the extra cycles being absorbed in the refresh free update below, we need to update again
-		gen->refresh_counter = gen->refresh_counter % interval;
-		gen->refresh_counter += delay;
-		delay += REFRESH_DELAY * MCLKS_PER_68K * (gen->refresh_counter / interval);
-		context->cycles += delay;
-	}
+	//Add refresh delays for any accesses that happened before the current one
+	context->cycles += refresh_advance(gen, before - gen->last_sync_cycle);
 	gen->last_sync_cycle = context->cycles;
 	//advance refresh counter for the current access, but don't generate delays
 	gen->refresh_counter += 4*MCLKS_PER_68K;
-	gen->refresh_counter = gen->refresh_counter % interval;
+	gen->refresh_counter = gen->refresh_counter % (MCLKS_PER_68K * REFRESH_INTERVAL);
 }
 
 void gen_update_refresh_no_wait(m68k_context *context)
@@ -678,14 +688,14 @@ static m68k_context *sync_components(m68k_context * context, uint32_t address)
 			}
 #endif
 		}
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 		if (gen->header.save_state) {
 #else
 		if (gen->header.save_state && (z_context->pc || !z_context->native_pc || z_context->reset || !z_context->busreq)) {
 #endif
 			uint8_t slot = gen->header.save_state - 1;
 			gen->header.save_state = 0;
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 			if (z_context->native_pc && !z_context->reset) {
 				//advance Z80 core to the start of an instruction
 				while (!z_context->pc)
@@ -1023,7 +1033,7 @@ static void enter_z80_debugger_immediately(void *context)
 	if (gen->z80->sync_cycle > gen->z80->Z80_CYCLE + 1) {
 		gen->z80->sync_cycle = gen->z80->Z80_CYCLE + 1;
 	}
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	if (gen->z80->target_cycle > gen->z80->sync_cycle) {
 		gen->z80->target_cycle = gen->z80->sync_cycle;
 	}
@@ -2907,7 +2917,7 @@ static genesis_context *shared_init(uint32_t system_opts, rom_info *rom, uint8_t
 	z80_options *z_opts = malloc(sizeof(z80_options));
 	init_z80_opts(z_opts, z80_map, 5, NULL, 0, MCLKS_PER_Z80, 0xFFFF);
 	gen->z80 = init_z80_context(z_opts);
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	gen->z80->next_int_pulse = z80_next_int_pulse;
 #endif
 	z80_assert_reset(gen->z80, 0);
diff --git a/genesis.h b/genesis.h
index ebb3fbb..f1326fb 100644
--- a/genesis.h
+++ b/genesis.h
@@ -10,9 +10,12 @@
 #include "system.h"
 #ifdef NEW_CORE
 #include "m68k.h"
-#include "z80.h"
 #else
 #include "m68k_core.h"
+#endif
+#ifdef NEW_Z80
+#include "z80.h"
+#else
 #include "z80_to_x86.h"
 #endif
 #include "ym2612.h"
diff --git a/gst.c b/gst.c
index 30c0d96..d955bf5 100644
--- a/gst.c
+++ b/gst.c
@@ -183,7 +183,7 @@ uint8_t z80_load_gst(z80_context * context, FILE * gstfile)
 	}
 	uint8_t * curpos = regdata;
 	uint8_t f = *(curpos++);
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	context->main[6] = context->last_flag_result = f;
 	context->chflags = ((f & 1) ? 0x80 : 0) | ((f & 0x10) ? 8 : 0);
 	context->nflag = f & 2;
@@ -226,7 +226,7 @@ uint8_t z80_load_gst(z80_context * context, FILE * gstfile)
 	context->sp = read_le_16(curpos);
 	curpos += 4;
 	f = *(curpos++);
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	context->alt[6] = f;
 	context->alt[7] = *curpos;
 	curpos += 3;
@@ -281,12 +281,12 @@ uint8_t z80_load_gst(z80_context * context, FILE * gstfile)
 	{
 		if (context->mem_pointers[0][i] != buffer[i]) {
 			context->mem_pointers[0][i] = buffer[i];
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 			z80_handle_code_write(i, context);
 #endif
 		}
 	}
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	context->native_pc = NULL;
 	context->extra_pc = NULL;
 #endif
@@ -371,7 +371,7 @@ uint8_t z80_save_gst(z80_context * context, FILE * gstfile)
 	uint8_t regdata[GST_Z80_REG_SIZE];
 	uint8_t * curpos = regdata;
 	memset(regdata, 0, sizeof(regdata));
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	uint8_t f = context->last_flag_result;
 	if (context->zflag) { f |= 0x40; }
 	if (context->chflags & 8) { f |= 0x10; }
@@ -417,7 +417,7 @@ uint8_t z80_save_gst(z80_context * context, FILE * gstfile)
 	curpos += 4;
 	write_le_16(curpos, context->sp);
 	curpos += 4;
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	*(curpos++) = context->alt[6];
 	*curpos = context->alt[7];
 	
diff --git a/sms.c b/sms.c
index dd59f85..b54a2df 100644
--- a/sms.c
+++ b/sms.c
@@ -11,7 +11,7 @@
 #include "bindings.h"
 #include "korean_sms_multi.h"
 
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 #define Z80_CYCLE cycles
 #define Z80_OPTS opts
 #define z80_handle_code_write(...)
@@ -81,7 +81,7 @@ static void update_interrupts(sms_context *sms)
 {
 	uint32_t vint = vdp_next_vint(sms->vdp);
 	uint32_t hint = vdp_next_hint(sms->vdp);
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 	sms->z80->int_cycle = vint < hint ? vint : hint;
 	z80_sync_cycle(sms->z80, sms->z80->sync_cycle);
 #else
@@ -1081,7 +1081,7 @@ static uint8_t load_state(system_header *system, uint8_t slot)
 	sms_context *sms = (sms_context *)system;
 	char *statepath = get_slot_name(system, slot, "state");
 	uint8_t ret;
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	if (!sms->z80->native_pc) {
 		ret = get_modification_time(statepath) != 0;
 		if (ret) {
@@ -1133,7 +1133,7 @@ static void run_sms(system_header *system)
 				}
 			}
 		}
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 		if ((system->enter_debugger || sms->z80->wp_hit) && sms->z80->pc) {
 			if (!sms->z80->wp_hit) {
 				system->enter_debugger = 0;
@@ -1143,7 +1143,7 @@ static void run_sms(system_header *system)
 #endif
 		}
 #endif
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 		if (sms->z80->nmi_cycle == CYCLE_NEVER) {
 #else
 		if (sms->z80->nmi_start == CYCLE_NEVER) {
@@ -1154,7 +1154,7 @@ static void run_sms(system_header *system)
 			}
 		}
 
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 		if (system->enter_debugger || sms->z80->wp_hit) {
 			target_cycle = sms->z80->Z80_CYCLE + 1;
 		}
@@ -1254,7 +1254,7 @@ static void soft_reset(system_header *system)
 {
 	sms_context *sms = (sms_context *)system;
 	z80_assert_reset(sms->z80, sms->z80->Z80_CYCLE);
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	sms->z80->target_cycle = sms->z80->sync_cycle = sms->z80->Z80_CYCLE;
 #endif
 }
@@ -1279,7 +1279,7 @@ static void request_exit(system_header *system)
 {
 	sms_context *sms = (sms_context *)system;
 	sms->should_return = 1;
-#ifndef NEW_CORE
+#ifndef NEW_Z80
 	sms->z80->target_cycle = sms->z80->sync_cycle = sms->z80->Z80_CYCLE;
 #endif
 }
diff --git a/sms.h b/sms.h
index f2efed4..e3092c3 100644
--- a/sms.h
+++ b/sms.h
@@ -4,7 +4,7 @@
 #include "system.h"
 #include "vdp.h"
 #include "psg.h"
-#ifdef NEW_CORE
+#ifdef NEW_Z80
 #include "z80.h"
 #else
 #include "z80_to_x86.h"
diff --git a/vdp.c b/vdp.c
index a8247ce..91edce3 100644
--- a/vdp.c
+++ b/vdp.c
@@ -3661,12 +3661,26 @@ static void vdp_h40_line(vdp_context * context)
 			}
 		}
 	}
-	advance_output_line(context);
 	//169-242 (inclusive)
 	for (int i = 0; i < 27; i++)
 	{
+		if (169 + i == BG_START_SLOT + LINEBUF_SIZE/2) {
+			//the slot-at-a-time path advances the output line here, not before
+			//the whole run of sprite slots
+			advance_output_line(context);
+			if (!context->output) {
+				context->output = dummy_buffer;
+			}
+		}
 		render_sprite_cells(context);
 		scan_sprite_table(context->vcounter, context);
+		if (i == 16) {
+			//the loop walks slots 169-182 (i 0-13) and then 229-242 (i 14-26),
+			//so i 16 is slot 231. Slot 232 is an external slot rather than a
+			//sprite render slot, which is why this runs 27 times over 28 slots,
+			//but the external slot itself still has to happen.
+			external_slot(context);
+		}
 	}
 	//243
 	render_sprite_cells(context);
@@ -3694,8 +3708,8 @@ static void vdp_h40_line(vdp_context * context)
 	context->hscroll_b = context->vdpmem[address+2] << 8 | context->vdpmem[address+3];
 	context->hscroll_b_fine = context->hscroll_b & 0xF;
 	//printf("%d: HScroll A: %d, HScroll B: %d\n", context->vcounter, context->hscroll_a, context->hscroll_b);
-	//243-246 inclusive
-	for (int i = 0; i < 3; i++)
+	//245-246 inclusive
+	for (int i = 0; i < 2; i++)
 	{
 		render_sprite_cells(context);
 		scan_sprite_table(context->vcounter, context);
@@ -3723,19 +3737,27 @@ static void vdp_h40_line(vdp_context * context)
 	scan_sprite_table(context->vcounter, context);
 	context->buf_a_off = (context->buf_a_off + SCROLL_BUFFER_DRAW) & SCROLL_BUFFER_MASK;
 	context->buf_b_off = (context->buf_b_off + SCROLL_BUFFER_DRAW) & SCROLL_BUFFER_MASK;
+	//249
+	read_map_scroll_a(0, context->vcounter, context);
 	//250
 	render_sprite_cells(context);
 	scan_sprite_table(context->vcounter, context);
 	//251
+	render_map_1(context);
 	scan_sprite_table(context->vcounter, context);//Just a guess
 	//252
+	render_map_2(context);
 	scan_sprite_table(context->vcounter, context);//Just a guess
+	//253
+	read_map_scroll_b(0, context->vcounter, context);
 	//254
 	render_sprite_cells(context);
 	scan_sprite_table(context->vcounter, context);
 	//255
+	render_map_3(context);
 	scan_sprite_table(context->vcounter, context);
 	//0
+	render_map_output(context->vcounter, 0, context);
 	scan_sprite_table(context->vcounter, context);//Just a guess
 	//seems like the sprite table scan fills a shift register
 	//values are FIFO, but unused slots precede used slots
@@ -3744,21 +3766,29 @@ static void vdp_h40_line(vdp_context * context)
 	context->cur_slot = context->slot_counter;
 	context->sprite_x_offset = 0;
 	context->sprite_draws = MAX_SPRITES_LINE;
-	//background planes and layer compositing
-	for (int col = 0; col < 42; col+=2)
+	//background planes and layer compositing, with sprite rendering phase 2
+	//interleaved exactly as COLUMN_RENDER_BLOCK does it: one read_sprite_x per
+	//column between read_map_scroll_b and render_map_3. Batching them after the
+	//loop instead changes which line a sprite ends up attributed to.
+	//Column 0 is rendered by the tail-of-line slots above, so this starts at 2.
+	for (int col = 2; col < 42; col+=2)
 	{
 		read_map_scroll_a(col, context->vcounter, context);
+		if (col & 7) {
+			//COLUMN_RENDER_BLOCK has an external slot here; the refresh variant
+			//used for every fourth column does not
+			external_slot(context);
+		}
 		render_map_1(context);
 		render_map_2(context);
 		read_map_scroll_b(col, context->vcounter, context);
+		read_sprite_x(context->vcounter, context);
 		render_map_3(context);
 		render_map_output(context->vcounter, col, context);
 	}
-	//sprite rendering phase 2
-	for (int i = 0; i < MAX_SPRITES_LINE; i++)
-	{
-		read_sprite_x(context->vcounter, context);
-	}
+	//161-162
+	external_slot(context);
+	external_slot(context);
 	//163
 	context->cur_slot = MAX_SPRITES_LINE-1;
 	memset(context->linebuf, 0, LINEBUF_SIZE);