added snow effect due to CPU accessing video memory at same time as VDG

Author: Commander Coder

6502.js

@@ -350,6 +350,10 @@ function Tube6502(model, cpu) {
         if (this.romPaged && (offset & 0xf000) === 0xf000) {
             return this.rom[offset & 0xfff];
         }
+
+        // video generator needs to know when CPU is accessing memory so it can make snow
+        if (model.isAtom && model.snow) this.video.video6847.cpuAddrAccess(offset & 0xffff);
+
         return this.memory[offset & 0xffff];
     };
     this.readmemZpStack = function (offset) {
@@ -833,6 +837,10 @@ export function Cpu6502(model, dbgr, video_, soundChip_, ddNoise_, music5000_, c
         if (this._debugWrite) this._debugWrite(addr, b);
         if (this.memStat[this.memStatOffset + (addr >>> 8)] === 1) {
             var offset = this.memLook[this.memStatOffset + (addr >>> 8)];
+
+            // video generator needs to know when CPU is accessing memory so it can make snow
+            if (model.isAtom && model.snow) this.video.video6847.cpuAddrAccess(offset + addr);
+
             this.ramRomOs[offset + addr] = b;
             return;
         }

6847.js

@@ -178,9 +178,38 @@ export function Video6847(video) {
         this.bitmapY = 0;
     };
 
+    /* 
+    Snow is caused by the CPU changing the address bus for 500ns at the 
+    same time as the VDG is expecting to get data from the address it has
+    requested, which takes 1100ns (1.1us).  This only occurs if the cpu is
+    access graphics memory.  So while the VDG is generating a scanline, the address
+    will change for 500ns based on the CPU memory accesses (read or write) and
+    the VDG will read 'noise' from the CPU addressed memory location instead.
+    */
+
+    this.cpuAddr = 0;
+    this.cpuAddrAccess = function (addr) {
+        // CPU has read from memory here
+        // VDG can read from this address for a cycle if it was video memory
+        // to generate snow
+        this.cpuAddr = addr;
+    };
+
     // atom video memory is 0x8000->0x9fff (8k but only bottom 6k used)
+    // effecively goes up to 0x9800
     this.readVideoMem = function () {
-        var memAddr = this.addr & 0x1fff; //6k
+        let cpuaddr = this.cpuAddr;
+
+        this.cpuAddr = 0; // reset the memory access by cpu but if it tries again it'll be set again
+
+        // during a vdg cycle, cpu might be active
+        if (this.vdg_cycles >= 0 && this.vdg_cycles < 1) {
+            if (cpuaddr > 0x8000 && this.cpuAddr <= 0x9800) {
+                return this.cpu.videoRead(cpuaddr);
+            }
+        }
+
+        var memAddr = this.addr & 0x1fff;
         memAddr |= 0x8000;
         return this.cpu.videoRead(memAddr);
     };

config.js

@@ -13,6 +13,8 @@ export function Config(onClose) {
         this.set65c02(this.model.tube);
         this.setTeletext(this.model.hasTeletextAdaptor);
         this.setMusic5000(this.model.hasMusic5000);
+        this.setSnowstorm(this.model.snow);
+        setModelMenus(this.model.name);
     });
 
     $configuration.addEventListener("hide.bs.modal", () => onClose(changed));
@@ -46,17 +48,34 @@ export function Config(onClose) {
         this.addRemoveROM("ats-3.0.rom", enabled);
     };
 
+    this.setSnowstorm = function (enabled) {
+        enabled = !!enabled;
+        $("#showAtomSnowstorm").prop("checked", enabled);
+        this.model.snow = enabled;
+    };
+
     function setDropdownText(modelName) {
         $("#bbc-model-dropdown .bbc-model").text(modelName);
     }
 
+    function setModelMenus(modelname) {
+        // set BBC and ATOM stuff
+        let atom = modelname.includes("Atom");
+
+        $("#65c02").prop("disabled", atom);
+        $("#hasTeletextAdaptor").prop("disabled", atom);
+        $("#hasMusic5000").prop("disabled", atom);
+        $("#showAtomSnowstorm").prop("disabled", !atom);
+    }
+
     $(".model-menu a").on(
         "click",
         function (e) {
             const modelName = $(e.target).attr("data-target");
             changed.model = modelName;
 
             setDropdownText($(e.target).text());
+            setModelMenus(modelName);
         }.bind(this)
     );
 
@@ -81,6 +100,13 @@ export function Config(onClose) {
         }.bind(this)
     );
 
+    $("#showAtomSnowstorm").on(
+        "click",
+        function () {
+            changed.snow = $("#showAtomSnowstorm").prop("checked");
+        }.bind(this)
+    );
+
     $(".keyboard-menu a").on(
         "click",
         function (e) {

index.html

@@ -934,6 +934,10 @@ <h5 class="modal-title" id="configurationModalLabel">Emulation Configuration</h5
                     <input class="form-check-input" type="checkbox" id="hasTeletextAdaptor" name="hasTeletextAdaptor" />
                     <label class="form-check-label" for="hasTeletextAdaptor">Teletext adaptor</label>
                   </div>
+                  <div class="form-check">
+                    <input class="form-check-input" type="checkbox" id="showAtomSnowstorm" name="showAtomSnowstorm" />
+                    <label class="form-check-label" for="showAtomSnowstorm">Atom Snowstorm</label>
+                  </div>
                 </div>
               </div>
             </div>

main.js

@@ -166,7 +166,8 @@ var config = new Config(function (changed) {
         changed.model ||
         changed.coProcessor !== undefined ||
         changed.hasMusic5000 !== undefined ||
-        changed.hasTeletextAdaptor !== undefined
+        changed.hasTeletextAdaptor !== undefined ||
+        changed.snow !== undefined
     ) {
         areYouSure(
             "Changing model requires a restart of the emulator. Restart now?",
@@ -193,6 +194,7 @@ config.setKeyLayout(keyLayout);
 config.set65c02(parsedQuery.coProcessor);
 config.setMusic5000(parsedQuery.hasMusic5000);
 config.setTeletext(parsedQuery.hasTeletextAdaptor);
+config.setSnowstorm(parsedQuery.snow);
 
 model = config.model;