Add register 0x04 for device commands

docs/Usage.md

@@ -24,11 +24,13 @@ When enabled this will:
    "Colors" originally created by NTSC artifacting will appear as sequences of dots.
  * Switch to the monochrome color palette (black & white by default)
 
+This setting will be saved across power-cycles if the "Save" command is executed.
+
 
 ## Device Registers
 
-The card's device registers can be written to change some settings. *These settings are
-not saved between power-cycles*
+The card's device registers can be written to change some settings. These settings persist across
+CPU resets and will be saved permanently if the "Save" command is executed (see register 4).
 
 The register base address depends on the slot number in which you installed the card:
 
@@ -92,3 +94,34 @@ For example, to replace the '#' character (character number `$A3`) with '£':
     POKE BASEADDR+3, 132
     POKE BASEADDR+3, 250
     POKE BASEADDR+3, 128
+
+Changes made to the text character patterns will be saved permanently when the "Save" command
+is executed.
+
+
+### Register 4 - Device command
+
+**[write-only]**
+This register allows a one-shot device command to be executed. The value written is the command
+to execute.
+
+|   value   | Description
+| --------- | -----------
+|    $00    | Load the default configuration values
+|    $01    | Load the stored configuration from flash. This automatically happens on power-up.
+|    $02    | Save the current configuration to flash
+
+
+
+#### Saving settings permanently
+
+To permanently save changes to flash, like say you update a text character pattern or want to always
+have scanline emulation enabled, then you just execute command `$02`:
+
+    POKE BASEADDR+4, 2
+
+To permanently restore the default settings (text font, monochrome colors, etc) you would need to execute
+command `$00` to revert the RAM settings to the defaults and then `$02` to save the change to flash:
+
+    POKE BASEADDR+4, 0
+    POKE BASEADDR+4, 2

pico/device_regs.c

@@ -1,7 +1,10 @@
 #include "device_regs.h"
 
+#include <string.h>
 #include "buffers.h"
 #include "colors.h"
+#include "config.h"
+#include "textfont.h"
 
 
 static unsigned int char_write_offset;
@@ -38,6 +41,37 @@ void device_write(uint_fast8_t reg, uint_fast8_t data) {
         char_write_offset = (char_write_offset + 1) % sizeof(character_rom);
         break;
 
+    // device command
+    case 0x04:
+        execute_device_command(data);
+        break;
+
+    default:;
+    }
+}
+
+
+// Handle a write to the "command" register to perform some one-shot action based on the
+// command value.
+//
+// Note: some of these commands could take a long time (relative to 6502 bus cycles) so
+// some bus activity may be missed. Other projects like the Analog-V2 delegate this execution
+// to the other (VGA) core to avoid this. Maybe do this if the missed bus cycles become a noticable
+// issue; I only expect it would happen when some config is being saved, which is not done often.
+void execute_device_command(uint_fast8_t cmd) {
+    switch(cmd) {
+    case 0x00:
+        // reset to the default configuration
+        config_load_defaults();
+        break;
+    case 0x01:
+        // reset to the saved configuration
+        config_load();
+        break;
+    case 0x02:
+        // save the current configuration
+        config_save();
+        break;
     default:;
     }
 }

pico/device_regs.h

@@ -3,3 +3,4 @@
 #include <stdint.h>
 
 extern void device_write(uint_fast8_t reg, uint_fast8_t data);
+extern void execute_device_command(uint_fast8_t cmd);