Rework bank switch and memory mode into their own fields.

This is to better define and respect the various modes that you can have
in Apple II memory access. This is a work in progress at the moment.

include/apple2.h

@@ -91,10 +91,93 @@ enum lores_colors {
  * flag below.
  */
 enum memory_mode {
-    MEMORY_ROM = 1,         // on = read ROM; off = read RAM
-    MEMORY_WRITE = 2,       // on = allow writes to RAM; off = disallow writes
-    MEMORY_RAM2 = 4,        // on = use bank 2 for $D000-$DFFF; off = use bank 1
-    MEMORY_AUX = 8,         // on = use auxiliary memory; off = use main memory
+    /*
+     * By default, memory accesses go to main memory in _all_ cases.
+     * Auxiliary memory is not used in any capacity.
+     */
+    MEMORY_DEFAULT = 0x0,
+
+    /*
+     * When this is on, the core 48k (non bank-switchable) of memory
+     * will be read from auxiliary memory. When off, it will be read
+     * from main memory.
+     */
+    MEMORY_READ_AUX = 0x1,
+    
+    /*
+     * When on, writes to the core 48k of memory will go to aux; when
+     * off, they go to main.
+     */
+    MEMORY_WRITE_AUX = 0x2,
+
+    /*
+     * This bit is what the tech reference calls an "enabling" switch,
+     * for PAGE2 and HIRES below. If this bit is not on, then those two
+     * other bits don't do anything, and all aux memory access is
+     * governed by WRITE_AUX and READ_AUX above.
+     */
+    MEMORY_80STORE = 0x4,
+
+    /*
+     * When 80STORE is on, PAGE2 will allow you to access auxiliary
+     * memory for the display page. The range depends on HIRES below.
+     * When PAGE2 is on and HIRES is off, then PAGE2 causes accesses to
+     * $0400..$07FF to always go to auxiliary memory (read or writes).
+     * When both PAGE2 and HIRES are on, then $2000..$3FFF also go to
+     * aux memory. When 80STORE is off, then these two bits are ignored.
+     */
+    MEMORY_PAGE2 = 0x8,
+    MEMORY_HIRES = 0x10,
+};
+
+enum bank_switch {
+    /*
+     * In nominal bank-switch mode, reads in the bank-switchable address
+     * space go to ROM; writes to RAM are protected; and bank2 memory is
+     * used.
+     */
+    BANK_DEFAULT = 0x0,
+
+    /* 
+     * When on, this reads from RAM in bank-switched memory. When off,
+     * it reads from ROM.
+     */
+    BANK_RAM = 0x1,
+    
+    /*
+     * When on, we will write to RAM. When off, we will write-protect
+     * RAM in bank-switched memory. NOTE: we can never write to ROM--or
+     * else it wouldn't be ROM! So if you have BANK_RAM off, but
+     * BANK_WRITE on, then writes do not fail, but they do go to RAM.
+     */
+    BANK_WRITE = 0x2,
+
+    /*
+     * When on, the $Dnnn hexapage will use the first RAM bank for
+     * reads and/or writes. All other RAM access in bank-switched memory
+     * will still go to bank 1 RAM, with respect to BANK_RAM when it
+     * comes to reads. (The default behavior is actually to use bank 2
+     * RAM for the $D range.)
+     */
+    BANK_RAM1 = 0x4,
+    
+    /*
+     * This is a weird little bit. When BANK_ALTZP is on, the zero page
+     * and stack are accessed from auxiliary memory rather than main
+     * memory. Those two pages of memory, however,  are _copied_ from one
+     * to the other, so data should remain consistent.
+     *
+     * That's not the weird part. That part makes sense given the name
+     * (which isn't my name, but is the name used in the IIe technical
+     * reference). The part that isn't so obvious is that
+     * bank-switchable RAM will _also_ be accessed from auxiliary
+     * memory, not main memory. Note that aux memory has its own second
+     * bank of RAM, the way that main memory does, so BANK_RAM2 works
+     * the way you think, but it works with the aux RAM2. No data is
+     * copied between main and aux's bank-switched memory, unlike the
+     * way zero page and the stack are handled.
+     */
+    BANK_ALTZP = 0x8,
 };
 
 typedef struct {
@@ -158,6 +241,13 @@ typedef struct {
      */
     vm_8bit bank_switch;
 
+    /*
+     * Beside bank-switching, we also need to keep track of memory
+     * modes; these pertain mostly to reading from main or auxiliary
+     * memory.
+     */
+    vm_8bit memory_mode;
+
     /*
      * Our two disk drives.
      */

src/apple2.c

@@ -57,7 +57,8 @@ apple2_create(int width, int height)
     // need to hard-set these values because apple2_set_bank_switch
     // assumes that the bank_switch variable has been initialized
     // before, which to this point, it hasn't!
-    mach->bank_switch = MEMORY_ROM | MEMORY_WRITE | MEMORY_RAM2;
+    mach->bank_switch = BANK_DEFAULT;
+    mach->memory_mode = MEMORY_DEFAULT;
 
     mach->main = vm_segment_create(APPLE2_MEMORY_SIZE);
     if (mach->main == NULL) {
@@ -251,7 +252,8 @@ apple2_reset(apple2 *mach)
     // - read from ROM
     // - write to RAM
     // - use bank 2 for $Dxxx hexapage
-    apple2_set_bank_switch(mach, MEMORY_ROM | MEMORY_WRITE | MEMORY_RAM2);
+    apple2_set_bank_switch(mach, BANK_DEFAULT);
+    apple2_set_memory_mode(mach, MEMORY_NOMINAL);
 }
 
 /*