Add int_FAC1

src/basic/,stubs_math/bccc.int_FAC1.s

@@ -1,19 +0,0 @@
-;; #LAYOUT# STD *       #TAKE
-;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-;; #LAYOUT# *   BASIC_0 #TAKE
-;; #LAYOUT# *   *       #IGNORE
-
-;
-; Math package - rounds away the decimal to make an integer
-;
-; See also:
-; - [CM64] Computes Mapping the Commodore 64 - page 116
-; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-;
-
-; XXX provide implementation
-
-int_FAC1:
-
-	+STUB_IMPLEMENTATION

src/basic/math/bccc.int_FAC1.jmp.s

@@ -0,0 +1,8 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+; Just a compatibility jump - our routine is longer than original one
+
+	jmp int_FAC1

src/basic/math/int_FAC1.s

@@ -0,0 +1,65 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - rounds away the decimal to make an integer
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 116
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+int_FAC1:
+
+    lda FAC1_exponent
+    cmp #$81
+    bcc int_FAC1_zero           ; exponent is 0 or lower so integer part is 0
+    cmp #$A0
+    bcs int_FAC1_out            ; exponent is 32 or larger so the entire mantissa is part of the integer
+
+    tax
+    and #$07                    ; low three bits of exponent is number of bits to keep
+    tay
+    
+    txa
+    lsr                         ; Bits 4-5 of exponent is how many full mantissa bytes to keep
+    lsr
+    lsr
+    and #$03
+    tax
+
+    lda #$FF                    ; Create bit mask for first mantissa byte to change
+@1: lsr
+    dey
+    bne @1
+    eor #$FF
+
+    and FAC1_mantissa,X
+    sta FAC1_mantissa,X
+    lda #0
+@2: inx
+    cpx #$4
+    bcs @3
+    sta FAC1_mantissa,X
+    bcc @2
+
+@3: lda FAC1_sign
+    bpl int_FAC1_out
+    lda #<const_ONE             ; If negative subtract one
+    ldy #>const_ONE
+    jsr sub_MEM_FAC1            ; FAC1 = 1 - FAC1
+    lda #$FF
+    sta FAC1_sign               ; FAC1 = FAC1 - 1
+    rts
+
+
+    ; FIXME: Do we need to set FACOV to 0?
+
+int_FAC1_zero:
+    lda #0
+    sta FAC1_exponent
+int_FAC1_out:
+    rts