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changeset 2336:83f5529086c5

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Make bogus word RAM read more appropriately bogus
author Michael Pavone <pavone@retrodev.com>
date Thu, 31 Aug 2023 20:28:33 -0700
parents c05b7c5e6f11
children 0e3118325c1c
files genesis.c segacd.h vdp.c vdp.h
diffstat 4 files changed, 21 insertions(+), 1 deletions(-) [+]
line wrap: on
line diff
--- a/genesis.c	Sat Aug 26 20:32:53 2023 -0700
+++ b/genesis.c	Thu Aug 31 20:28:33 2023 -0700
@@ -285,13 +285,29 @@
 		uint32_t word_ram_end = cd->base + 0x240000;
 		if (address >= word_ram && address < word_ram_end) {
 			//FIXME: first word should just be garbage
-			address -= 2;
+			if (!cd->has_vdp_dma_value) {
+				cd->vdp_dma_value = read_word(genesis->m68k->last_prefetch_address, (void **)genesis->m68k->mem_pointers, &genesis->m68k->options->gen, genesis->m68k);
+				cd->has_vdp_dma_value = 1;
+			}
+			uint16_t ret = cd->vdp_dma_value;
+			cd->vdp_dma_value = read_word(address, (void **)genesis->m68k->mem_pointers, &genesis->m68k->options->gen, genesis->m68k);
+			return ret;
 		}
 	}
 
 	return read_word(address, (void **)genesis->m68k->mem_pointers, &genesis->m68k->options->gen, genesis->m68k);
 }
 
+void vdp_dma_started(void)
+{
+	genesis_context *genesis = (genesis_context *)current_system;
+	if (genesis->expansion) {
+		segacd_context *cd = genesis->expansion;
+		cd->has_vdp_dma_value = 0;
+	}
+}
+
+
 static uint16_t get_open_bus_value(system_header *system)
 {
 	genesis_context *genesis = (genesis_context *)system;
--- a/segacd.h	Sat Aug 26 20:32:53 2023 -0700
+++ b/segacd.h	Thu Aug 31 20:28:33 2023 -0700
@@ -30,6 +30,7 @@
 	uint32_t        graphics_dx;
 	uint32_t        graphics_dy;
 	uint16_t        graphics_dst_x;
+	uint16_t        vdp_dma_value;
 	uint8_t         graphics_pixels[4];
 	uint8_t         graphics_debug_window;
 	uint8_t         timer_pending;
@@ -50,6 +51,7 @@
 	uint8_t         sub_paused_wordram;
 	uint8_t         bram_cart_write_enabled;
 	uint8_t         bram_cart_id;
+	uint8_t         has_vdp_dma_value;
 	rf5c164         pcm;
 	lc8951          cdc;
 	cdd_mcu         cdd;
--- a/vdp.c	Sat Aug 26 20:32:53 2023 -0700
+++ b/vdp.c	Thu Aug 31 20:28:33 2023 -0700
@@ -4635,6 +4635,7 @@
 					//sticking with 3 slots for now until I can do some more captures
 					vdp_run_context_full(context, context->cycles + 12 * ((context->regs[REG_MODE_2] & BIT_MODE_5) && (context->regs[REG_MODE_4] & BIT_H40) ? 4 : 5));
 					context->flags |= FLAG_DMA_RUN;
+					vdp_dma_started();
 					return 1;
 				} else {
 					context->flags |= FLAG_DMA_RUN;
--- a/vdp.h	Sat Aug 26 20:32:53 2023 -0700
+++ b/vdp.h	Thu Aug 31 20:28:33 2023 -0700
@@ -304,6 +304,7 @@
 void vdp_inc_debug_mode(vdp_context *context);
 //to be implemented by the host system
 uint16_t read_dma_value(uint32_t address);
+void vdp_dma_started(void);
 void vdp_replay_event(vdp_context *context, uint8_t event, event_reader *reader);
 
 #endif //VDP_H_