Add shiftr_FAC

src/basic/math/b983.shiftr_FAC.s

@@ -0,0 +1,65 @@
+;; #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 a floating point register right
+;
+; The main entry point is shiftr_FAC which shifts any FAC to the right with sign extension
+; Inputs:
+; - A is the negative of the number of bits to shift
+; - X points to to the bytes to be shifted
+; Returns:
+; - Y = 0
+; - CF = 0
+; - A = overflowed bits
+;
+; Alternative entry point shift_for_mul_RES will shift the RES register for multiplication.
+; Alternative entry point short_shift_FAC will makes a short shift without sign extension
+;
+; This is verified to be identical to the original Microsoft implementation where it was named SHIFTR.
+;
+
+shift_for_mul_RES:
+    ldx #RESHO-1
+
+.bytes:
+    ldy 4,X             ; Shift entire bytes
+    sty FACOV
+    ldy 3,X
+    sty 4,X
+    ldy 2,X
+    sty 3,X
+    ldy 1,X
+    sty 2,X
+    ldy BITS
+    sty 1,X
+
+shiftr_FAC:
+    adc #8
+    bmi .bytes          ; More than or equal to 8 bits remaining
+    beq .bytes
+    sbc #8              ; Go back to previous value
+    tay
+    lda FACOV
+    bcs shiftr_FAC
+.sext:
+    asl 1,X
+    bcc @2
+    inc 1,X
+@2:
+    ror 1,X             ; Undo previous shift
+    ror 1,X
+short_shift_FAC:
+    ror 2,X
+    ror 3,X
+    ror 4,X
+    ror
+    iny
+    bne .sext
+shftrt:
+    clc
+    rts