Add support for unions

Unions allow multiple memory allocations (using ds, etc.) to share the
same space in memory.

This allows games to use the same memory for different purposes,
depending on their state.

This also adds documentation on how to use the new UNION, NEXTU, and
ENDU keywords.

src/asm/asmy.y

@@ -431,6 +431,34 @@ void	if_skip_to_endc()
 	nLineNo--;
 }
 
+void startUnion() {
+	if (!pCurrentSection) {
+		fatalerror("UNIONs must be inside a SECTION");
+	}
+	
+	ULONG unionIndex = nUnionDepth;
+	nUnionDepth++;
+	if (nUnionDepth > MAXUNIONS) {
+		fatalerror("Too many nested UNIONs");
+	}
+	
+	unionStart[unionIndex] = nPC;
+	unionSize[unionIndex] = 0;
+}
+
+void updateUnion() {
+	ULONG unionIndex = nUnionDepth - 1;
+	ULONG size = nPC - unionStart[unionIndex];
+	
+	if (size > unionSize[unionIndex]) {
+		unionSize[unionIndex] = size;
+	}
+	
+	nPC = unionStart[unionIndex];
+	pCurrentSection->nPC = unionStart[unionIndex];
+	pPCSymbol->nValue = unionStart[unionIndex];
+}
+
 %}
 
 %union
@@ -506,6 +534,7 @@ void	if_skip_to_endc()
 %token	T_POP_MACRO
 %token	T_POP_ENDM
 %token	T_POP_RSRESET T_POP_RSSET
+%token	T_POP_UNION T_POP_NEXTU T_POP_ENDU
 %token	T_POP_INCBIN T_POP_REPT
 %token	T_POP_CHARMAP
 %token	T_POP_SHIFT
@@ -644,6 +673,9 @@ simple_pseudoop	:	include
 				|	section
 				|	rsreset
 				|	rsset
+				|	union
+				|	nextu
+				|	endu
 				|	incbin
 				|	charmap
 				|	rept
@@ -745,6 +777,31 @@ rb				:	T_LABEL T_POP_RB uconst
 					}
 ;
 
+union			:	T_POP_UNION {
+						startUnion();
+					};
+
+nextu			:	T_POP_NEXTU {
+						if (nUnionDepth <= 0) {
+							fatalerror("Found NEXTU outside of a UNION construct");
+						}
+	
+						updateUnion();
+					};
+
+endu			:	T_POP_ENDU {
+						if (nUnionDepth <= 0) {
+							fatalerror("Found ENDU outside of a UNION construct");
+						}
+						
+						updateUnion();
+	
+						nUnionDepth--;
+						nPC = unionStart[nUnionDepth] + unionSize[nUnionDepth];
+						pCurrentSection->nPC = nPC;
+						pPCSymbol->nValue = nPC;
+					};
+
 ds				:	T_POP_DS uconst
 					{ out_Skip( $2 ); }
 ;