--- a/jag_video.c	Fri Oct 07 18:27:38 2016 -0700
+++ b/jag_video.c	Sat Oct 08 23:49:20 2016 -0700
@@ -1,7 +1,23 @@
 #include <stdint.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include "jag_video.h"
 
+enum {
+	VMODE_CRY,
+	VMODE_RGB24,
+	VMODE_DIRECT16,
+	VMODE_RGB16,
+	VMODE_VARIABLE
+};
+
+char *vmode_names[] = {
+	"CRY",
+	"RGB16",
+	"DIRECT16",
+	"VARIABLE"
+};
+
 jag_video *jag_video_init(void)
 {
 	return calloc(1, sizeof(jag_video));
@@ -10,4 +26,78 @@
 void jag_video_run(jag_video *context, uint32_t target_cycle)
 {
 	context->cycles = target_cycle;
+	
 }
+
+static uint8_t is_reg_writeable(uint32_t address)
+{
+	return address < VID_HLPEN || address >= VID_OBJLIST1;
+}
+
+void jag_video_reg_write(jag_video *context, uint32_t address, uint16_t value)
+{
+	uint32_t reg = (address >> 1 & 0x7F) - 2;
+	if (reg < JAG_VIDEO_REGS && is_reg_writeable(reg)) {
+		context->regs[reg] = value;
+		if (reg == VID_VMODE) {
+			context->pclock_div = (value >> 9 & 7) + 1;
+			context->pclock_counter = 0;
+			if (value & 0x10) {
+				context->mode = VMODE_VARIABLE;
+			} else {
+				context->mode = value >> 1 & 3;
+			}
+			printf("Mode %s, pixel clock divider: %d, time base generation: %s\n", vmode_names[context->mode], context->pclock_div, value & 1 ? "enabled" : "disabled");
+		}
+		switch (reg)
+		{
+		case VID_OBJLIST1:
+			printf("Object List Pointer 1: %X\n", value);
+			break;
+		case VID_OBJLIST2:
+			printf("Object List Pointer 2: %X\n", value);
+			break;
+		case VID_HPERIOD:
+			printf("Horizontal period: %d\n", value & 0x3FF);
+			break;
+		case VID_HBLANK_BEGIN:
+			printf("horizontal blanking begin: %d\n", value & 0x7FF);
+			break;
+		case VID_HBLANK_END:
+			printf("horizontal blanking end: %d\n", value & 0x7FF);
+			break;
+		case VID_HSYNC:
+			printf("horizontal sync start: %d\n", value & 0x7FF);
+			break;
+		case VID_HDISP_BEGIN1:
+			printf("horizontal display begin 1: %d\n", value & 0x7FF);
+			break;
+		case VID_HDISP_BEGIN2:
+			printf("horizontal display begin 2: %d\n", value & 0x7FF);
+			break;
+		case VID_HDISP_END:
+			printf("horizontal display end: %d\n", value & 0x7FF);
+			break;
+		case VID_VPERIOD:
+			printf("Vertical period: %d\n", value & 0x7FF);
+			break;
+		case VID_VBLANK_BEGIN:
+			printf("vertical blanking begin: %d\n", value & 0x7FF);
+			break;
+		case VID_VBLANK_END:
+			printf("vertical blanking end: %d\n", value & 0x7FF);
+			break;
+		case VID_VSYNC:
+			printf("vertical sync start: %d\n", value & 0x7FF);
+			break;
+		case VID_VDISP_BEGIN:
+			printf("vertical display begin: %d\n", value & 0x7FF);
+			break;
+		case VID_VDISP_END:
+			printf("vertical display end: %d\n", value & 0x7FF);
+			break;
+		}
+	} else {
+		fprintf(stderr, "Write to invalid video/object processor register %X:%X\n", address, value);
+	}
+}