added FAC1 denormalization code, for division

c64/basic/math/div_FAC1_denorm.s

@@ -0,0 +1,70 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - denormalize FAC1 mantissa and adapt its exponent accordingly,
+// so that division is more precise
+//
+
+// XXX test this
+
+div_FAC1_denorm:
+
+	// Start by moving whole bytes, if possible
+
+	lda FACOV
+	bne div_FAC1_denorm_by_bit         // branch if last byte of mantissa is non-zero
+
+	lda FAC1_exponent
+	cmp #$09
+	bcc div_FAC1_denorm_by_bit         // branch if not suitable to decrement exponent by 8
+
+	// Decrement exponent
+
+	sbc #$08                           // Carry already set here
+	sta FAC1_exponent
+
+	// Move mantissa bytes
+
+	lda FAC1_mantissa+3
+	sta FACOV
+	lda FAC1_mantissa+2
+	sta FAC1_mantissa+3
+	lda FAC1_mantissa+1
+	sta FAC1_mantissa+2	
+	lda FAC1_mantissa+0
+	sta FAC1_mantissa+1
+	lda #$00
+	sta FAC1_mantissa+0
+
+	beq div_FAC1_denorm                // branch always
+
+div_FAC1_denorm_by_bit:
+
+	lda FACOV
+	and #$01
+	bne div_FAC1_denorm_done           // branch if lowest bit of mantissa is already 1
+
+	lda FAC1_exponent
+	cmp #$02
+	bcc div_FAC1_denorm_done           // branch if not suitable to decrement exponent
+
+	// Decrement exponent
+
+	dec FAC1_exponent
+
+	// Move mantissa bits
+
+	clc
+	ror FAC1_mantissa+0
+	ror FAC1_mantissa+1
+	ror FAC1_mantissa+2
+	ror FAC1_mantissa+3
+	ror FACOV
+
+	bcc div_FAC1_denorm_done           // branch always
+
+div_FAC1_denorm_done:
+
+	rts