implemented MOVFM math routine

c64/basic/math/ba8c.CONUPK.s

@@ -11,10 +11,10 @@
 //
 // Output:
 // - .A - FAC1 exponent (yes, really - Codebase64 is right)
-// - .Y - XXX 0?
+// - .Y - 0 (experimentation with original ROM)
 //
 // Preserves:
-// - .X - (experimentation with original routine)
+// - .X - (experimentation with original ROM)
 //
 // See also:
 // - [CM64] Computes Mapping the Commodore 64 - page 114
@@ -26,11 +26,11 @@
 
 CONUPK:
 
-	// Not sure which temporary variables are used for storage by the original ROMs,
-	// but $22/$23 (INDEX+0 and INDEX+1) seem to be good candidates, at least some
-	// math routines use them as pointer storage, see:
+	// Checked in the available documentation:
 	// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
 	// - https://www.lemon64.com/forum/viewtopic.php?t=67576&sid=d41e77cdddd28d6f9c9d52ce5b4e8dc3
+	// that $22/$23 (INDEX+0 and INDEX+1) are at least sometimes used for pointer storage.
+	// Confirmed by experimenting with original ROM that this is the case here too.
 
 	sty INDEX+1
 	sta INDEX+0
@@ -38,8 +38,6 @@ CONUPK:
 	// Now copy the data from RAM to FAC2, for the format description see:
 	// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic#Representation_in_the_C-64
 
-	ldy #$04
-
 	// Copy the mantissa - reverse byte order
 
 	ldy #$04

c64/basic/math/bba2.MOVFM.s

@@ -0,0 +1,85 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+// Math package - fetch FAC1 from RAM location
+//
+// Input:
+// - .A - address low byte
+// - .Y - address high byte
+//
+// Output:
+// - .A - FAC1 exponent, affects status flags
+// - .Y - always 0
+//
+// Preserves:
+// - .X - (experimentation with original ROM)
+//
+// See also:
+// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+
+// XXX test this
+
+MOVMF:
+
+	// Checked in the available documentation:
+	// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+	// - https://www.lemon64.com/forum/viewtopic.php?t=67576&sid=d41e77cdddd28d6f9c9d52ce5b4e8dc3
+	// that $22/$23 (INDEX+0 and INDEX+1) are at least sometimes used for pointer storage.
+	// Confirmed by experimenting with original ROM that this is the case here too.
+
+	sty INDEX+1
+	sta INDEX+0
+
+	// Now copy the data from RAM to FAC2, for the format description see:
+	// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic#Representation_in_the_C-64
+
+	// Copy the mantissa - reverse byte order
+
+	ldy #$04
+	lda (INDEX), y
+	sta FAC1_mantissa+3
+
+	dey
+	lda (INDEX), y
+	sta FAC1_mantissa+2
+
+	dey
+	lda (INDEX), y
+	sta FAC1_mantissa+1
+
+	dey
+	lda (INDEX), y
+	ora #$80                           // clear the sign bit
+	sta FAC1_mantissa+0
+
+	// Copy the sign
+
+	lda (INDEX), y
+	bpl !+                             // assumption: non-negative numbers are more frequent
+	lda #$FF
+	skip_2_bytes_trash_nvz
+!:
+	lda $00
+
+	sta FAC1_sign
+
+	// Copy the exponent
+
+	dey
+	lda (INDEX), y
+	sta FAC1_exponent
+
+	// Set low order mantissa - to my surprise original ROM seems to put 0 to FACOW - XXX why???
+	// Since we do not know the original value (lost due to variable format having one byte shorter
+    // mantissa than FAC1), it is probably best to put some approximate value here; putting $80
+    // would be risky in case rouding is performed later, $7F seems to be sane choice
+
+	lda #$7F
+	sta FACOV
+
+	// Return the FAC1 exponent
+
+	lda FAC1_exponent
+	rts

c64/basic/math/bba2.interop

@@ -0,0 +1,3 @@
+This is an important floating point math routine, documented in many places.
+
+XXX - try to find real-life example