some work towards floating point division

CREDITS.md

@@ -7,6 +7,7 @@
 * Graham - for his PAL/NTSC detection routine
 * enthusi/Onslaught - for his example Turbo Tape 64 compatible loader
 * djmips - for fast 8x8 multiplication routine
+* Verz - for integer division routine
 
 ## for documentation
 

STATUS.md

@@ -205,7 +205,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BAE2`   | `mul10_FAC1`           | NOT DONE |                                                    |
 | `$BAFE`   | `div10_FAC1_p`         | NOT DONE |                                                    |
 | `$BB0F`   | `div_MEM_FAC1`         | PARTIAL  | needs `div_FAC2_FAC1`                              |
-| `$BB12`   | `div_FAC2_FAC1`        | NOT DONE |                                                    |
+| `$BB12`   | `div_FAC2_FAC1`        | NOT DONE | some work already started                          |
 | `$BBA2`   | `mov_MEM_FAC1`         | DONE     |                                                    |
 | `$BBA6`   | `get_FAC2_via_INDEX`   | NOT DONE |                                                    |
 | `$BBC7`   | `mov_r_FAC1_TMP2`      | PARTIAL  | not fully tested yet                               |

c64/basic/,stubs_math/bb12.div_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 - divide FAC2 by FAC1, ignores sign
-//
-// Input:
-// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
-//
-// Output:
-// - leaves the quotient FAC1 and the remainder in FAC2
-//
-// See also:
-// - [CM64] Computes Mapping the Commodore 64 - page 114
-// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-//
-
-// XXX provide implementation
-
-div_FAC2_FAC1:
-	STUB_IMPLEMENTATION()

c64/basic/math/bb12.div_FAC2_FAC1.s

@@ -0,0 +1,108 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - divide FAC2 (dividend) by FAC1 (divisor), ignores sign
+//
+// Input:
+// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
+//
+// Output:
+// - leaves the quotient FAC1 and the remainder in FAC2
+//
+// See also:
+// - [CM64] Computes Mapping the Commodore 64 - page 114
+// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+//
+
+// XXX provide implementation
+
+div_FAC2_FAC1:
+	STUB_IMPLEMENTATION()
+
+
+
+
+
+
+// XXX move this to a separate file
+
+// Unsigned integer 32-bit mantissa division, based on code by Verz
+// divident: FAC2_mantissa, divisor: FAC1_mantissa, result: FAC2_mantissa, remainder: RESHO
+// temporary storage: INDEX+1, INDEX+2, INDEX+3
+//
+// https://codebase64.org/doku.php?id=base:24bit_division_24-bit_result
+
+// XXX test this, optimize (for temporary storage use .Y, .Z and RESHO+4)
+
+div_mantissas:
+
+	// Preset remainder to 0
+	// XXX should be common with multiplication
+
+	lda #$00
+	sta RESHO+0
+	sta RESHO+1
+	sta RESHO+2
+	sta RESHO+3
+
+	ldx #$32                           // repeat for each bit
+
+	// FALLTROUGH
+
+div_mantissas_loop:
+
+	// Multiply divident by 2, msb -> Carry
+
+	asl FAC2_mantissa+3
+	rol FAC2_mantissa+2	
+	rol FAC2_mantissa+1
+	rol FAC2_mantissa+0
+	
+	// Multiply remainder by 2, lsb <- Carry
+
+	rol RESHO+3
+	rol RESHO+2
+	rol RESHO+1
+	rol RESHO+0
+
+	// Subtract divisor to see if it fits in
+
+	lda RESHO+3
+	sec
+	sbc FAC1_mantissa+2
+	sta INDEX+3
+	
+	lda RESHO+2
+	sbc FAC1_mantissa+2
+	sta INDEX+2
+
+	lda RESHO+1
+	sbc FAC1_mantissa+1
+	sta INDEX+1
+
+	lda RESHO+0
+	sbc FAC1_mantissa+0
+	bcc div_mantissas_skip             // if Carry is not set then divisor did not fit in yet
+
+	sta RESHO+0                        // else save substraction result as new remainder,
+	lda INDEX+1
+	sta RESHO+1
+	lda INDEX+2
+	sta RESHO+2
+	lda INDEX+3
+	sta RESHO+3
+
+	inc FAC2_mantissa+3                // and INCrement result cause divisor fit in 1 times
+
+	// FALLTROUGH
+
+div_mantissas_skip:
+
+	dex
+	bne div_mantissas_loop
+
+	rts