Fix USCF Chess and Land Battle: enable $D000-$D3FF RAM 8 (#97)

* Fix USCF Chess and Land Battle: enable D000-D3FF RAM 8

USCF Chess (1981) and Land Battle (1982) both use load method 4,
which requires 8-bit RAM at \-\. writeMem() was silently
discarding all writes to this range, causing the game's CPU to be
unable to store piece positions, BACKTAB data, or working variables.
This produced completely garbled graphics (GROM character garbage
instead of chess pieces/board tiles).

Fix:
- Add d000_ram flag to memory.h/memory.c
- writeMem: allow 8-bit writes to \-\ when d000_ram is set
- readMem: mask reads from \-\ to 8 bits when d000_ram is set
- d000_ram is reset at start of MemoryInit() (on each cart load)
- load4() in cart.c now sets d000_ram = 1 instead of the TODO comment

Also fix C89 compliance in stic.c: replace // comments with /* */

Fixes: #96

* Add screenshots/Thumbs.db to .gitignore

---------

Co-authored-by: Jason Carr <jcarr71@users.noreply.github.com>

Author: jcarr71

.gitignore

@@ -39,3 +39,4 @@ build-test.ps1
 *.temp
 *.log
 Assets/Thumbs.db
+screenshots/Thumbs.db

src/cart.c

@@ -209,7 +209,7 @@ void load3()
 void load4()
 {
 	loadRange(0x5000, 0x6FFF);
-	// [memattr] $D000 - $D3FF = RAM 8 // automatic 
+	d000_ram = 1; /* $D000-$D3FF = RAM 8 */
 }
 
 void load5()

src/memory.c

@@ -25,6 +25,8 @@
 
 unsigned int Memory[0x10000];
 
+int d000_ram = 0; /* 1 = $D000-$D3FF is 8-bit RAM (e.g. USCF Chess) */
+
 int stic_and[64] = {
     0x07ff, 0x07ff, 0x07ff, 0x07ff, 0x07ff, 0x07ff, 0x07ff, 0x07ff,
     0x0fff, 0x0fff, 0x0fff, 0x0fff, 0x0fff, 0x0fff, 0x0fff, 0x0fff,
@@ -66,6 +68,10 @@ void writeMem(int adr, int val) // Write (should handle hooks/alias)
         case 0x15:  /* A800-AFFF */
         case 0x16:  /* B000-B7FF */
         case 0x1a:  /* D000-D7FF */
+            if (d000_ram && adr <= 0xD3FF) {
+                Memory[adr] = val & 0xFF; /* RAM 8 */
+            }
+            return;
         case 0x1b:  /* D800-DFFF */
         case 0x1c:  /* E000-E7FF */
         case 0x1d:  /* E800-EFFF */
@@ -146,13 +152,19 @@ int readMem(int adr) // Read (should handle hooks/alias)
 		val = val & 0xFF;
 	}
 
+	if(d000_ram && adr>=0xD000 && adr<=0xD3FF)
+	{
+		val = val & 0xFF; /* RAM 8 */
+	}
+
 	return val;
 }
 
 void MemoryInit()
 {
 	int i;
-	for(i=0x0000; i<=0x0007; i++) { Memory[i] = 0x3800; } // STIC Registers
+	d000_ram = 0; /* reset per-cart flags before loading new cart */
+	for(i=0x0000; i<=0x0007; i++) { Memory[i] = 0x3800; } /* STIC Registers */
 	for(i=0x0008; i<=0x000F; i++) { Memory[i] = 0x3000; }
 	for(i=0x0010; i<=0x0017; i++) { Memory[i] = 0x0000; }
 	for(i=0x0018; i<=0x001F; i++) { Memory[i] = 0x3C00; }
@@ -171,6 +183,6 @@ void MemoryInit()
 	for(i=0x4000; i<=0x4FFF; i++) { Memory[i] = 0xFFFF; }
 	for(i=0x5000; i<=0x5FFF; i++) { Memory[i] = 0x0000; }
 	for(i=0x6000; i<=0xFFFF; i++) { Memory[i] = 0xFFFF; }
-	Memory[0x1FE] = 0xFF; // Controller R
-	Memory[0x1FF] = 0xFF; // Controller L
+	Memory[0x1FE] = 0xFF; /* Controller R */
+	Memory[0x1FF] = 0xFF; /* Controller L */
 }

src/memory.h

@@ -20,6 +20,8 @@
 
 extern unsigned int Memory[0x10000];
 
+extern int d000_ram; /* 1 = $D000-$D3FF is 8-bit RAM (e.g. USCF Chess) */
+
 void MemoryInit(void);
 
 int readMem(int adr);

src/stic.c

@@ -361,9 +361,9 @@ void drawBackgroundColorStack(int scanline)
             fgcolor = fgcard[col];
             bgcolor = bgcard[col];
 
-            if (((card >> 11) & 0x01) != 0) // Limit GRAM to 64 cards
+            if (((card >> 11) & 0x01) != 0) /* Card is from GRAM - limit to 64 cards */
                 gaddress = 0x3000 + (card & 0x09f8);
-            else
+            else                             /* Card is from GROM */
                 gaddress = 0x3000 + (card & 0x0ff8);
 
             gdata = Memory[gaddress + cardrow]; // fetch current line of current card graphic