Add rom, ram2 bank memory; modify read/write sig

The signature for read/write map functions now accepts a void pointer so
that we can pass in the machine that those functions may need to know
about. This is a bit hairy, but allows us to avoid the need for a
yet-uglier global variable.

include/apple2.h

@@ -6,6 +6,21 @@
 #include "vm_bitfont.h"
 #include "vm_screen.h"
 
+/*
+ * The size of our block of ROM is 12k
+ */
+#define APPLE2_ROM_SIZE 0x3000
+
+/*
+ * Whereas the second bank of RAM is a mere 4k
+ */
+#define APPLE2_RAM2_SIZE 0x1000
+
+/*
+ * This is the base address (or offset) for all bank-switched memory
+ */
+#define APPLE2_BANK_OFFSET 0xD000
+
 enum video_modes {
     VIDEO_40COL_TEXT,
     VIDEO_LORES,
@@ -41,6 +56,17 @@ enum lores_colors {
     LORES_WHITE,
 };
 
+/*
+ * These are the potential memory modes we understand. You can only have
+ * one memory mode at a time.
+ */
+enum memory_mode {
+    MEMORY_BANK_ROM,        // the last 12k is system ROM
+    MEMORY_BANK_RAM1,       // the last 12k is system RAM
+    MEMORY_BANK_RAM2,       // the first 4k of the last 12k is a separate RAM
+                            // block from that in RAM1
+};
+
 typedef struct {
     /*
      * The apple 2 hardware used an MOS-6502 processor.
@@ -54,6 +80,21 @@ typedef struct {
      */
     vm_segment *memory;
 
+    /*
+     * The Apple II used a system of bank-switched memory to enable
+     * software to address a separate block of ROM.
+     */
+    vm_segment *rom;
+    
+    /*
+     * Additionally, the Apple II had a standalone block of RAM (with no
+     * good name for it, really, hence the regrettably vague "ram2") so
+     * that you technically could use 16k of RAM from a set of 12k
+     * addresses. The extra 4k lives a lonely life in the garage
+     * apartment.
+     */
+    vm_segment *ram2;
+
     /*
      * The screen wherein we shall render all of our graphics.
      */
@@ -80,6 +121,13 @@ typedef struct {
      */
     int color_mode;
 
+    /*
+     * This describes the behavior of our bank-switching scheme. We need
+     * our read/write mappers to know where writes into the
+     * bank-switched area of memory should target.
+     */
+    int memory_mode;
+
     /*
      * Our two disk drives.
      */

src/apple2.c

@@ -39,6 +39,8 @@ apple2_create(int width, int height)
 
     // Forward set these to NULL in case we fail to build the machine
     // properly; that way, we won't try to free garbage data
+    mach->rom = NULL;
+    mach->ram2 = NULL;
     mach->sysfont = NULL;
     mach->screen = NULL;
     mach->drive1 = NULL;
@@ -51,8 +53,18 @@ apple2_create(int width, int height)
         return NULL;
     }
 
+    // Our memory is that which is owned by the CPU.
     mach->memory = mach->cpu->memory;
 
+    // Initliaze our system ROM and separate bank-switched block of RAM
+    mach->rom = vm_segment_create(APPLE2_ROM_SIZE);
+    mach->ram2 = vm_segment_create(APPLE2_RAM2_SIZE);
+    if (mach->rom == NULL || mach->ram2 == NULL) {
+        log_critical("Could not initialize ROM / RAM2!");
+        apple2_free(mach);
+        return NULL;
+    }
+
     // Our two drives -- we create both of them, even if we intend to
     // use only one.
     mach->drive1 = apple2dd_create();
@@ -179,6 +191,14 @@ apple2_free(apple2 *mach)
         mos6502_free(mach->cpu);
     }
 
+    if (mach->rom) {
+        vm_segment_free(mach->rom);
+    }
+
+    if (mach->ram2) {
+        vm_segment_free(mach->ram2);
+    }
+
     if (mach->sysfont) {
         vm_bitfont_free(mach->sysfont);
     }