From 508a9aedf03d887dcc00a8c53caf8cef1ada2937 Mon Sep 17 00:00:00 2001
From: Michael Pavone <pavone@retrodev.com>
Date: Sun, 24 Apr 2016 02:19:48 -0700
Subject: Half assed, prefetch based open bus value emulation. Gets BlastEm up
 to 119/122 in VDP FIFO Testing

---
 68kinst.c       |  2 ++
 68kinst.h       |  1 +
 blastem.c       | 11 +++++++----
 blastem.h       |  1 +
 m68k_core.c     |  7 +++++++
 m68k_core.h     |  1 +
 m68k_core_x86.c |  5 +++++
 m68k_internal.h |  1 +
 vdp.c           |  3 +--
 9 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/68kinst.c b/68kinst.c
index 64aa5df..b1a2f37 100644
--- a/68kinst.c
+++ b/68kinst.c
@@ -1546,8 +1546,10 @@ uint16_t * m68k_decode(uint16_t * istream, m68kinst * decoded, uint32_t address)
 	}
 	if (decoded->op == M68K_INVALID) {
 		decoded->src.params.immed = *start;
+		decoded->bytes = 2;
 		return start + 1;
 	}
+	decoded->bytes = 2 * (istream + 1 - start);
 	return istream+1;
 }
 
diff --git a/68kinst.h b/68kinst.h
index 0d5072f..cab2574 100644
--- a/68kinst.h
+++ b/68kinst.h
@@ -282,6 +282,7 @@ typedef struct m68kinst {
 		uint8_t size;
 		uint8_t cond;
 	} extra;
+	uint8_t bytes;
 	uint32_t address;
 	m68k_op_info src;
 	m68k_op_info dst;
diff --git a/blastem.c b/blastem.c
index 2601c0e..354fb04 100644
--- a/blastem.c
+++ b/blastem.c
@@ -144,6 +144,11 @@ uint16_t read_dma_value(uint32_t address)
 	return 0;
 }
 
+uint16_t get_open_bus_value()
+{
+	return read_dma_value(genesis->m68k->last_prefetch_address/2);
+}
+
 void adjust_int_cycle(m68k_context * context, vdp_context * v_context)
 {
 	//static int old_int_cycle = CYCLE_NEVER;
@@ -705,8 +710,7 @@ uint8_t io_read(uint32_t location, m68k_context * context)
 		} else {
 			if (location == 0x1100) {
 				value = z80_enabled ? !z80_get_busack(gen->z80, context->current_cycle) : !gen->z80->busack;
-				//TODO: actual pre-fetch emulation
-				value |= 0x4E;
+				value |= (get_open_bus_value() >> 8) & 0xFE;
 				dprintf("Byte read of BUSREQ returned %d @ %d (reset: %d)\n", value, context->current_cycle, gen->z80->reset);
 			} else if (location == 0x1200) {
 				value = !gen->z80->reset;
@@ -726,8 +730,7 @@ uint16_t io_read_w(uint32_t location, m68k_context * context)
 		value = value | (value << 8);
 	} else {
 		value <<= 8;
-		//TODO: actual pre-fetch emulation
-		value |= 0x73;
+		value |= get_open_bus_value() & 0xFF;
 	}
 	return value;
 }
diff --git a/blastem.h b/blastem.h
index 70c5134..eb1cc74 100644
--- a/blastem.h
+++ b/blastem.h
@@ -68,6 +68,7 @@ extern uint16_t *ram;
 extern uint8_t z80_ram[Z80_RAM_BYTES];
 
 uint16_t read_dma_value(uint32_t address);
+uint16_t get_open_bus_value();
 m68k_context * sync_components(m68k_context *context, uint32_t address);
 m68k_context * debugger(m68k_context * context, uint32_t address);
 void set_speed_percent(genesis_context * context, uint32_t percent);
diff --git a/m68k_core.c b/m68k_core.c
index a39830b..c920768 100644
--- a/m68k_core.c
+++ b/m68k_core.c
@@ -796,6 +796,13 @@ void translate_m68k(m68k_options * opts, m68kinst * inst)
 {
 	check_cycles_int(&opts->gen, inst->address);
 	//log_address(&opts->gen, inst->address, "M68K: %X @ %d\n");
+	if (
+		(inst->src.addr_mode > MODE_AREG && inst->src.addr_mode < MODE_IMMEDIATE) 
+		|| (inst->dst.addr_mode > MODE_AREG && inst->dst.addr_mode < MODE_IMMEDIATE)
+	) {
+		//Not accurate for all cases, but probably good enough for now
+		m68k_set_last_prefetch(opts, inst->address + inst->bytes);
+	}
 	impl_info * info = m68k_impls + inst->op;
 	if (info->itype == RAW_FUNC) {
 		info->impl.raw(opts, inst);
diff --git a/m68k_core.h b/m68k_core.h
index 8c8deb6..7aeb3f3 100644
--- a/m68k_core.h
+++ b/m68k_core.h
@@ -57,6 +57,7 @@ typedef struct {
 	uint32_t        sync_cycle;
 	uint32_t        int_cycle;
 	uint32_t        int_num;
+	uint32_t        last_prefetch_address;
 	uint16_t        *mem_pointers[NUM_MEM_AREAS];
 	code_ptr        resume_pc;
 	native_map_slot *native_code_map;
diff --git a/m68k_core_x86.c b/m68k_core_x86.c
index 9882b35..149e84d 100644
--- a/m68k_core_x86.c
+++ b/m68k_core_x86.c
@@ -2167,6 +2167,11 @@ void translate_out_of_bounds(code_info *code)
 	call_args(code, (code_ptr)exit, 1, RDI);
 }
 
+void m68k_set_last_prefetch(m68k_options *opts, uint32_t address)
+{
+	mov_irdisp(&opts->gen.code, address, opts->gen.context_reg, offsetof(m68k_context, last_prefetch_address), SZ_D);
+}
+
 void nop_fill_or_jmp_next(code_info *code, code_ptr old_end, code_ptr next_inst)
 {
 	if (next_inst == old_end && next_inst - code->cur < 2) {
diff --git a/m68k_internal.h b/m68k_internal.h
index b74f06b..8b891bd 100644
--- a/m68k_internal.h
+++ b/m68k_internal.h
@@ -32,6 +32,7 @@ void calc_index_disp8(m68k_options *opts, m68k_op_info *op, uint8_t native_reg);
 void calc_areg_index_disp8(m68k_options *opts, m68k_op_info *op, uint8_t native_reg);
 void nop_fill_or_jmp_next(code_info *code, code_ptr old_end, code_ptr next_inst);
 void check_user_mode_swap_ssp_usp(m68k_options *opts);
+void m68k_set_last_prefetch(m68k_options *opts, uint32_t address);
 
 //functions implemented in m68k_core.c
 int8_t native_reg(m68k_op_info * op, m68k_options * opts);
diff --git a/vdp.c b/vdp.c
index 8b1ed22..5e1fab8 100644
--- a/vdp.c
+++ b/vdp.c
@@ -1694,9 +1694,8 @@ void vdp_test_port_write(vdp_context * context, uint16_t value)
 uint16_t vdp_control_port_read(vdp_context * context)
 {
 	context->flags &= ~FLAG_PENDING;
-	//TODO: Open bus emulation
 	//Bits 15-10 are not fixed like Charles MacDonald's doc suggests, but instead open bus values that reflect 68K prefetch
-	uint16_t value = 0;
+	uint16_t value = get_open_bus_value() & 0xFC00;
 	if (context->fifo_read < 0) {
 		value |= 0x200;
 	}
-- 
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