naming cleanup in math routines

c64/basic/math/round_FAC1.s

@@ -0,0 +1,61 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - rounds FAC1 before copying
+//
+// See also:
+// - [CM64] Computes Mapping the Commodore 64 - page 115
+// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+//
+
+// XXX test this
+
+round_FAC1:
+
+	// If number is 0, do not do anything
+
+	lda FAC1_exponent
+	beq round_FAC1_done
+
+	lda FACOV
+	bpl round_FAC1_zero_FACOV          // branch if it is enough to clear FACOV
+
+	// Increment the 4 bytes of matissa
+
+	// Curse you, who invented this format! Had the mantissa been little endian,
+	// we could have used INW on 65C02 and above, the code would have been smaller!
+
+	inc FAC1_mantissa+3
+	bne round_FAC1_zero_FACOV
+	inc FAC1_mantissa+2
+	bne round_FAC1_zero_FACOV
+	inc FAC1_mantissa+1
+	bne round_FAC1_zero_FACOV
+	inc FAC1_mantissa+0
+	bne round_FAC1_zero_FACOV
+
+	// If we are here, it means mantissa is 0; put it to minimum allowed and increment exponent
+
+	lda #$80
+	sta FAC1_mantissa+0
+	inc FAC1_exponent
+	bne round_FAC1_zero_FACOV
+
+	// If we are here, it means the exponent wrapped around to $00; put it back to $FF
+
+	dec FAC1_exponent
+
+	// FALLTROUGH
+
+round_FAC1_zero_FACOV:
+
+	lda #$00
+	sta FACOV
+
+	// FALLTROUGH
+
+round_FAC1_done:
+
+	rts