started work on pseudorandom number generator

STATUS.md

@@ -237,6 +237,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BFED`   | `exp_FAC1`             | NOT DONE |                                                    |
 | `$E043`   | `poly1_FAC1`           | NOT DONE |                                                    |
 | `$E059`   | `poly2_FAC1`           | NOT DONE |                                                    |
+| `$E0BE`   | `rnd_generate`         | DONE     |                                                    |
 | `$E264`   | `cos_FAC1`             | NOT DONE |                                                    |
 | `$E26B`   | `sin_FAC1`             | NOT DONE |                                                    |
 | `$E2B4`   | `tan_FAC1`             | NOT DONE |                                                    |

c64/aliases/,aliases_ram_lowmem.s

@@ -76,7 +76,7 @@
 	.label FBUFPT    = $71 // $71-$72  -- NOT IMPLEMENTED --
 	.label CHRGET    = $73 // $73-$8A  -- NOT IMPLEMENTED --
 	.label TXTPTR    = $7A // $7A-$7B  current BASIC statement pointer
-	.label RNDX      = $8B // $8B-$8F  -- NOT IMPLEMENTED --
+	.label RNDX      = $8B // $8B-$8F  random number seed
 
 	//
 	// Page 0 - Kernal area ($90-$FF)

c64/basic/math/e0be.interop

@@ -0,0 +1,5 @@
+This is an important floating point math routine, see:
+
+- https://www.atarimagazines.com/compute/issue72/random_numbers.php
+
+XXX - try to find real-life example

c64/basic/math/e0be.rnd_generate.s

@@ -0,0 +1,63 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// A fast random generator, idea from:
+// - https://codebase64.org/doku.php?id=base:32bit_galois_lfsr
+//
+// Output:
+// - FAC1
+//
+// See also:
+// - https://www.atarimagazines.com/compute/issue72/random_numbers.php
+//
+
+rnd_generate:
+
+	// Generate new 4-byte random number
+
+	asl RNDX+1
+	rol RNDX+2
+	rol RNDX+3
+	rol RNDX+4
+
+	bcs rnd_generate_copy
+
+	lda RNDX+1
+	eor #$B7
+	sta RNDX+1
+	lda RNDX+2
+	eor #$1D
+	sta RNDX+2
+	lda RNDX+3
+	eor #$C1
+	sta RNDX+3
+	lda RNDX+4
+	eor #$04
+	sta RNDX+4
+
+	// FALLTROUGH
+
+rnd_generate_copy:
+
+	// Copy the number to FAC1, order changed on purpose
+
+	lda RNDX+3
+	sta FAC1_mantissa+0
+	lda RNDX+1
+	sta FAC1_mantissa+1
+	lda RNDX+2
+	sta FAC1_mantissa+2
+	lda RNDX+4
+	sta FAC1_mantissa+3
+	
+	lda #$00
+	sta FACOV
+	sta FAC1_sign
+	
+	lda #$80
+	sta FAC1_exponent
+	sta RNDX+0                         // in case someone tries to use the float directly
+	
+	jmp normal_FAC1