Add link-time RST instruction

This allows using a label as the argument to a `rst` instruction
Fixes rednex#448

src/asm/asmy.y

@@ -2065,9 +2065,10 @@ z80_rrca	: T_Z80_RRCA
 
 z80_rst		: T_Z80_RST const_8bit
 		{
-			if (rpn_isReloc(&$2))
-				yyerror("Address for RST must be absolute");
-			else if (($2.nVal & 0x38) != $2.nVal)
+			if (rpn_isReloc(&$2)) {
+				rpn_CheckRST(&$2, &$2);
+				out_RelByte(&$2);
+			} else if (($2.nVal & 0x38) != $2.nVal)
 				yyerror("Invalid address $%x for RST", $2.nVal);
 			else
 				out_AbsByte(0xC7 | $2.nVal);

src/asm/rpn.c

@@ -242,6 +242,13 @@ void rpn_CheckHRAM(struct Expression *expr, const struct Expression *src)
 	expr->nRPNPatchSize++;
 }
 
+void rpn_CheckRST(struct Expression *expr, const struct Expression *src)
+{
+	*expr = *src;
+	pushbyte(expr, RPN_RST);
+	expr->nRPNPatchSize++;
+}
+
 void rpn_LOGNOT(struct Expression *expr, const struct Expression *src)
 {
 	*expr = *src;

src/link/patch.c

@@ -284,6 +284,17 @@ static int32_t computeRPNExpr(struct Patch const *patch,
 			value &= 0xFF;
 			break;
 
+		case RPN_RST:
+			value = popRPN();
+			/* Acceptable values are 0x00, 0x08, 0x10, ..., 0x38
+			 * They can be easily checked with a bitmask
+			 */
+			if (value & ~0x38)
+				errx(1, "%s(%d): Value %d is not a RST vector",
+				     patch->fileName, patch->lineNo, value);
+			value |= 0xC7;
+			break;
+
 		case RPN_CONST:
 			value = 0;
 			for (uint8_t shift = 0; shift < 32; shift += 8)

src/rgbds.5

@@ -170,7 +170,9 @@ Symbol ID follows.
 a null-terminated string follows.
 .It Li $52 Ta Li Current BANK()
 .It Li $60 Ta Li HRAMCheck .
-Checks if the value is in HRAM, AND it with 0xFF.
+Checks if the value is in HRAM, ANDs it with 0xFF.
+.It Li $61 Ta Li RSTCheck .
+Checks if the value is a RST vector, ORs it with 0xC7.
 .It Li $80 Ta Ar LONG
 integer follows.
 .It Li $81 Ta Ar LONG