fix for floating point subtraction, started work on multiplication

STATUS.md

@@ -199,7 +199,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B983`   | `MULSHF`               | NOT DONE |                                                    |
 | `$B9EA`   | `log_FAC1`             | NOT DONE |                                                    |
 | `$BA28`   | `mul_MEM_FAC1`         | PARTIAL  | needs `mul_FAC2_FAC1`                              |
-| `$BA2B`   | `mul_FAC2_FAC1`        | NOT DONE |                                                    |
+| `$BA2B`   | `mul_FAC2_FAC1`        | NOT DONE | some work started                                  |
 | `$BA59`   | `MLTPLY`               | NOT DONE |                                                    |
 | `$BA8C`   | `mov_MEM_FAC2`         | DONE     |                                                    |
 | `$BAE2`   | `mul10_FAC1`           | NOT DONE |                                                    |

c64/basic/,stubs_math/ba2b.mul_FAC2_FAC1.s

@@ -1,24 +0,0 @@
-// #LAYOUT# STD *       #TAKE
-// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-// #LAYOUT# *   BASIC_0 #TAKE
-// #LAYOUT# *   *       #IGNORE
-
-//
-// Math package - multiplies FAC1 by FAC2
-//
-// Input:
-// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
-//
-// Note:
-// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
-//
-// See also:
-// - [CM64] Computes Mapping the Commodore 64 - page 113 XXX address does not match
-// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-//
-
-// XXX provide implementation
-
-mul_FAC2_FAC1:
-	STUB_IMPLEMENTATION()

c64/basic/math/add_FAC2_FAC1_sub.s

@@ -12,7 +12,7 @@
 add_FAC2_FAC1_sub:
 
 	// First verify that |FAC1| > |FAC2|, branch to proper variant if it is not
-	// Note, that exponents are laready equalized
+	// We ignore FACOV here. Note, that exponents are already equalized
 
 	lda FAC1_mantissa+0
 	cmp FAC2_mantissa+0
@@ -59,9 +59,7 @@ add_FAC2_FAC1_sub:
 	sbc FAC2_mantissa+0
 	sta FAC1_mantissa+0	
 	
-	bcs add_FAC2_FAC1_sub_normalize    // end if no need to adapt the exponent
+	// Exponent should never need adaptation here
-
-	dec FAC1_exponent
 	
 	jmp normal_FAC1
 
@@ -94,14 +92,6 @@ add_FAC2_FAC1_sub_rev:
 	sbc FAC1_mantissa+0
 	sta FAC1_mantissa+0
 	
-	bcs add_FAC2_FAC1_sub_normalize    // end if no need to adapt the exponent
+	// Exponent should never need adaptation here
-	
-	lda FAC2_exponent
-	sta FAC1_exponent
-	dec FAC1_exponent
-	
-	// FALLTROUGH
-
-add_FAC2_FAC1_sub_normalize:
 
 	jmp normal_FAC1

c64/basic/math/b86a.add_FAC2_FAC1.s

@@ -67,6 +67,10 @@ add_FAC2_FAC1:
 	inc FAC1_exponent
 	bne add_FAC2_FAC1_adapt_mantissa   // end if no overflow
 	
+	// FALLTROUGH
+
+set_FAC1_max:                          // entry used by other routines too
+
 	lda #$FF
 	sta FAC1_exponent
 	sta FAC1_mantissa+3

c64/basic/math/ba2b.mul_FAC2_FAC1.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 mul_FAC2_FAC1

c64/basic/math/mov_MEM_FAC1.s

@@ -22,7 +22,7 @@
 // XXX test this
 
 mov_MEM_FAC1:
-.break
+
 	// Original routines use $22/$23 location for a vector too, see here:
 	// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
 

c64/basic/math/mul_FAC2_FAC1.s

@@ -0,0 +1,90 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - multiplies FAC1 by FAC2
+//
+// Input:
+// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
+//
+// Note:
+// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
+//
+// See also:
+// - [CM64] Computes Mapping the Commodore 64 - page 113 XXX address does not match
+// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+//
+
+// XXX provide implementation
+
+mul_FAC2_FAC1:
+
+	// First handle special case of multiplying by 0
+	
+	lda FAC1_exponent
+	beq mul_FAC2_FAC1_done
+	
+	lda FAC2_exponent
+	bne !+
+	sta FAC1_exponent
+	beq mul_FAC2_FAC1_done
+!:
+	// Multiply signs
+	
+	lda FAC1_sign
+	eor FAC2_sign
+	sta FAC1_sign
+	
+	// Check if exponents can be added without overflow/underflow
+	
+	lda FAC1_exponent
+	and #$80
+	eor FAC2_exponent
+	
+	bmi mul_FAC2_FAC1_add_exponents    // exponent signs are opposite, we can add them safely
+	
+	// Both exponents are either negative (below $80), or both are non-negative ($80 or above),
+	// try to add them
+	
+	lda FAC1_exponent
+	clc
+	adc FAC2_exponent
+	bpl mul_FAC2_FAC1_sub_exp_bias     // branch if we can add values
+	
+	// We cannot add them, result would over/underflow; set result as
+	// either 0 or maximum possible float
+	
+	lda FAC1_exponent
+	bmi_16 set_FAC1_max
+
+	lda #$00
+	sta FAC1_exponent
+	rts
+	
+mul_FAC2_FAC1_add_exponents:
+	
+	// Add the exponents
+	
+	lda FAC1_exponent
+	clc
+	adc FAC2_exponent
+	
+	// FALLTROUGH
+	
+mul_FAC2_FAC1_sub_exp_bias:
+	
+	sec
+	sbc #$80                           // bias
+	sta FAC1_exponent
+	
+	// Multiply the mantissas
+	
+	// XXX
+
+	STUB_IMPLEMENTATION()
+
+mul_FAC2_FAC1_done:
+
+	rts