Implement floating bank in linker script

Turns out that we can solve being unable to choose a bank to change
the PC of, by simply refusing to choose!

man/rgblink.5

@@ -86,6 +86,22 @@ If the bank has never been active thus far, the
 .Dq current address
 defaults to the beginning of the bank
 .Pq e.g. Ad $4000 No for Ic ROMX No sections .
+.Pp
+Instead of giving a bank number, the keyword
+.Ic FLOATING
+can be used instead; this sets the type of the subsequent sections without binding them to a particular bank.
+(If the type only allows a single bank, e.g.
+.Ic ROM0 ,
+then
+.Ic FLOATING
+is valid but redundant and has no effect.)
+Since no particular section is active, the
+.Dq current address
+is made floating (as if by a
+.Ql Ic FLOATING
+directive), and
+.Ic ORG
+is not allowed.
 .It Changing the current address
 A bank must be active for any of these directives to be used.
 .Pp
@@ -104,6 +120,7 @@ causes all sections between it and the next
 .Ic ORG
 or bank specification to be placed at addresses automatically determined by
 .Nm .
+.Pq It is, however, compatible with Ic ALIGN No below.
 .Pp
 .Ql Ic ALIGN Ar addr , Ar offset
 increases the

src/link/script.y

@@ -35,6 +35,7 @@
 	static void includeFile(std::string &&path);
 	static void incLineNo();
 
+	static void setFloatingSectionType(SectionType type);
 	static void setSectionType(SectionType type);
 	static void setSectionType(SectionType type, uint32_t bank);
 	static void setAddr(uint32_t addr);
@@ -111,6 +112,9 @@ directive:
 	| sect_type number {
 		setSectionType($1, $2);
 	}
+	| sect_type FLOATING {
+		setFloatingSectionType($1);
+	}
 	| FLOATING {
 		makeAddrFloating();
 	}
@@ -388,6 +392,20 @@ static void setActiveTypeAndIdx(SectionType type, uint32_t idx) {
 	}
 }
 
+static void setFloatingSectionType(SectionType type) {
+	if (nbbanks(type) == 1) {
+		setActiveTypeAndIdx(type, 0); // There is only a single bank anyway, so just set the index to 0.
+	} else {
+		activeType = type;
+		activeBankIdx = UINT32_MAX;
+		// Force PC to be floating for this kind of section.
+		// Because we wouldn't know how to index into `curAddr[activeType]`!
+		isPcFloating = true;
+		floatingAlignMask = 0;
+		floatingAlignOffset = 0;
+	}
+}
+
 static void setSectionType(SectionType type) {
 	auto const &context = lexerStack.back();
 
@@ -433,6 +451,10 @@ static void setAddr(uint32_t addr) {
 		scriptError(context, "Cannot set the current address: no memory region is active");
 		return;
 	}
+	if (activeBankIdx == UINT32_MAX) {
+		scriptError(context, "Cannot set the current address: the bank is floating");
+		return;
+	}
 
 	auto &pc = curAddr[activeType][activeBankIdx];
 	auto const &typeInfo = sectionTypeInfo[activeType];
@@ -627,6 +649,9 @@ static void placeSection(std::string const &name, bool isOptional) {
 		);
 	}
 
+	if (activeBankIdx == UINT32_MAX) {
+		section->isBankFixed = false;
+	} else {
 		uint32_t bank = activeBankIdx + typeInfo.firstBank;
 		if (section->isBankFixed && bank != section->bank) {
 			scriptError(
@@ -641,6 +666,7 @@ static void placeSection(std::string const &name, bool isOptional) {
 		}
 		section->isBankFixed = true;
 		section->bank = bank;
+	}
 
 	if (!isPcFloating) {
 		uint16_t &org = curAddr[activeType][activeBankIdx];

test/link/script-syntax-error.out

@@ -1,3 +1,3 @@
 error: script-syntax-error.link(2): syntax error, unexpected ORG, expecting newline or OPTIONAL
-error: script-syntax-error.link(5): syntax error, unexpected string, expecting newline or number
+error: script-syntax-error.link(5): syntax error, unexpected string, expecting newline or FLOATING or number
 Linking failed with 2 errors