Add inv_FAC1_mantissa

src/basic/math/b947.interop

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

src/basic/math/b947.inv_FAC1_mantissa.s

@@ -0,0 +1,36 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - invert FAC1 mantissa
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 113
+; - https://www.c64-wiki.com/wiki/BASIC-ROM
+; - https://sta.c64.org/cbm64basconv.html
+;
+
+; 2-complement the FAC1 mantissa
+; The 4 byte mantissa is treated as an integer in big endian form
+
+
+inv_FAC1_mantissa:
+
+    ldx #3
+@1: lda FAC1_mantissa,X
+    eor #$FF
+    sta FAC1_mantissa,X
+    dex
+    bpl @1
+
+    inc FAC1_mantissa+3
+    bne @2
+    inc FAC1_mantissa+2
+    bne @2
+    inc FAC1_mantissa+1
+    bne @2
+    inc FAC1_mantissa
+    
+@2: rts