increased floating point multiplication precision

c64/basic/math/mul_FAC2_FAC1_shift.s

@@ -8,6 +8,16 @@
 
 mul_FAC2_FAC1_shift:
 
+	// Check if rounding is needed
+
+	php
+	clc
+	lda RESHO+4
+	bpl !+
+	sec
+!:
+	// Copy bytes
+
 	lda RESHO+3
 	sta RESHO+4
 	lda RESHO+2
@@ -18,5 +28,19 @@ mul_FAC2_FAC1_shift:
 	sta RESHO+1
 	lda #$00
 	sta RESHO+0
+	bcc !+
 
+	// Perform rounding
+
+	inc RESHO+4
+	bne !+
+	inc RESHO+3
+	bne !+
+	inc RESHO+2
+	bne !+
+	inc RESHO+1
+	bne !+
+	inc RESHO+0
+!:
+	plp
 	rts