Add shift_carry_into_FAC1

src/basic/math/b936.shift_carry_into_FAC1.s

@@ -0,0 +1,34 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
+; Math package - shift carry into FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named SQUEEZ.
+;
+; Alternative entry point: RNDSHF to also shift in a CF=0
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 113
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+shift_carry_into_FAC1:
+
+SQUEEZ:
+    bcc RNDRTS
+RNDSHF:
+    inc FAC1_exponent
+    beq overflow_error
+    ror FAC1_mantissa+0
+    ror FAC1_mantissa+1
+    ror FAC1_mantissa+2
+    ror FAC1_mantissa+3
+    ror FACOV
+RNDRTS:
+    rts