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annotate src/vdp.c @ 43:6e7bfe83d2b0

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Changed the design to vastly simplify the video hardware and support a 23-bit address space on the CPU
author Michael Pavone <pavone@retrodev.com>
date Sat, 27 Aug 2016 22:38:31 -0700
parents 083347ccd508
children
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1 #include <stdint.h>
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2 #include <string.h>
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3 #include <stdio.h>
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4 #include "vdp.h"
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5 #include "system.h"
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6
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7 #define MAX_ACTIVE_LINES 240
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8 #define TOTAL_LINES 262
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9 #define ACTIVE_WIDTH 320
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10 #define TOTAL_WIDTH 416
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11
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12 #define VDP_STATUS_FB_SELECT 1
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13 #define VDP_STATUS_PENDING_VINT 2
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14 #define VDP_STATUS_VBLANK 4
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15 #define VDP_STATUS_CRAM_PENDING 8
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16 #define VDP_STATUS_VINT_ENABLED 0x2000
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17 #define VDP_STATUS_DEPTH 0x4000
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18 #define VDP_STATUS_ENABLED 0x8000
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19
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20 void vdp_init(vdp *context, uint32_t clock_div)
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21 {
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22 memset(context, 0, sizeof(vdp));
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23 //clock div specifies the pixel clock divider
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24 //but our emulation step is half that fast
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25 context->clock_inc = clock_div*2;
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26 }
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28 void vdp_run(vdp *context, uint32_t target)
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29 {
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30 uint8_t *current_fb = context->status & VDP_STATUS_FB_SELECT ? context->vram + 64*1024 : context->vram;
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31
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32 while (context->cycles < target)
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33 {
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34 context->hcounter+=2;
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35 if (context->hcounter == TOTAL_WIDTH) {
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36 context->hcounter = 0;
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37 context->vcounter++;
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38 if (context->vcounter == TOTAL_LINES) {
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39 context->vcounter = 0;
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40 }
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41 }
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42 //Draw to framebuffer
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43 if (context->vcounter < MAX_ACTIVE_LINES && context->hcounter < ACTIVE_WIDTH) {
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44 if (!context->framebuffer) {
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45 context->framebuffer = system_get_framebuffer(&context->pitch);
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46 //pitch is in terms of bytes, but we want it in terms of pixels
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47 context->pitch /= sizeof(uint16_t);
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48 }
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49 uint16_t *dest = context->framebuffer + context->vcounter * context->pitch + context->hcounter;
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50 if (
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51 context->status & VDP_STATUS_ENABLED
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52 && context->vcounter >= context->top_skip
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53 && context->vcounter < MAX_ACTIVE_LINES - context->bottom_skip
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54 ) {
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55 if (context->status & VDP_STATUS_DEPTH) {
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56 uint16_t offset = context->start_offset + (context->vcounter - context->top_skip) * ACTIVE_WIDTH + context->hcounter;
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57 //8bpp
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58 *(dest++) = context->cram[current_fb[offset++]];
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59 *dest = context->cram[current_fb[offset]];
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60 } else {
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61 //4bpp
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62 uint8_t pixels = current_fb[context->start_offset + (context->vcounter - context->top_skip) * ACTIVE_WIDTH + context->hcounter >> 1];
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63 *(dest++) = context->cram[context->pal_select | pixels >> 4];
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64 *dest = context->cram[context->pal_select | (pixels & 0xF)];
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65 }
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66 } else {
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67 *(dest++) = context->cram[0];
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68 *dest = context->cram[0];
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69 }
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70 } else if (context->framebuffer && context->hcounter < ACTIVE_WIDTH) {
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71 system_framebuffer_updated();
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72 context->framebuffer = NULL;
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73 }
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74 if (!context->hcounter) {
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75 if (context->vcounter == (context->vcounter - context->bottom_skip)) {
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76 context->status |= VDP_STATUS_PENDING_VINT | VDP_STATUS_VBLANK;
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77 } else if (context->vcounter == context->top_skip) {
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78 //clear pending interrupt flag since VBlank is over
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79 context->status &= ~(VDP_STATUS_PENDING_VINT | VDP_STATUS_VBLANK);
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80 }
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81 }
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82 context->cycles += context->clock_inc;
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83 }
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84 }
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85 void vdp_write_mode(vdp *context, uint16_t value)
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86 {
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87 uint16_t status_bits = VDP_STATUS_ENABLED | VDP_STATUS_DEPTH | VDP_STATUS_VINT_ENABLED | VDP_STATUS_FB_SELECT;
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88 context->status &= ~status_bits;
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89 context->status |= value & status_bits;
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90 context->pal_select = value >> 7 & 0x30;
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91 context->top_skip = value >> 6 & 0x1F;
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92 context->bottom_skip = value >> 1 & 0x1F;
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93 }
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94
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95 void vdp_write_cram(vdp *context, uint16_t value)
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96 {
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97 if (context->status & VDP_STATUS_CRAM_PENDING) {
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98 context->cram[context->pal_write_index++] = value;
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99 if (!(--context->pal_write_count)) {
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100 context->status &= ~VDP_STATUS_CRAM_PENDING;
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101 }
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102 } else {
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103 context->pal_write_count = value;
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104 context->pal_write_index = value >> 8;
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105 context->status |= VDP_STATUS_CRAM_PENDING;
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106 }
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107 }
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108
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109 uint32_t vdp_next_interrupt(vdp *context)
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110 {
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111 if (context->status & VDP_STATUS_PENDING_VINT) {
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112 return 0;
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113 } else if (context->status & VDP_STATUS_ENABLED) {
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114 uint32_t next_line = context->vcounter + 1;
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115 uint32_t next_line_cyc = context->cycles + ((TOTAL_WIDTH - context->hcounter) >> 1) * context->clock_inc;
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116 uint32_t vint_line = (MAX_ACTIVE_LINES - context->bottom_skip);
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117 if (context->vcounter < vint_line) {
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118 return next_line_cyc + (vint_line - next_line) * (TOTAL_WIDTH >> 1) * context->clock_inc;
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119 } else {
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120 return next_line_cyc + (vint_line + TOTAL_LINES - next_line) * (TOTAL_WIDTH >> 1) * context->clock_inc;
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121 }
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122 } else {
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123 return 0xFFFFFFFF;
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124 }
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125 }
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126
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127 void vdp_ack_interrupt(vdp *context)
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128 {
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129 context->status &= ~VDP_STATUS_PENDING_VINT;
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130 }
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131
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132 uint8_t vdp_interrupt_pending(vdp *context)
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133 {
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134 return (context->status & VDP_STATUS_PENDING_VINT) != 0;
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135 }
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136
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137 uint8_t *vdp_get_back_buffer(vdp *context)
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138 {
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139 return context->status & VDP_STATUS_FB_SELECT ? context->vram : context->vram + 64*1024;
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140 }