Add get_FAC1_via_INDEX

Also change to only use bit 7 as sign in FAC1 and FAC2 (as the original BASIC does) to simplify
implementations.

testsuite/unit/math.vs

@@ -139,12 +139,22 @@ r A=$E0
 r Y=$B9
 r PC=$BBA2
 ret
-fill $1001 $1006 80 80 00 00 00 FF
+fill $1001 $1006 80 80 00 00 00 80
 print "COMPARE"
 compare $61 $66 $1001
 compare $70 $70 $1003  ; FACOV = 0
 print "END"
 
+print "TEST: get_FAC1_via_INDEX -0.5"
+fill $61 $66 FF FF FF FF FF AA
+fill $22 $23 E0 B9
+r PC=$BBA6
+ret
+fill $1001 $1006 80 80 00 00 00 80
+print "COMPARE"
+compare $61 $66 $1001
+print "END"
+
 print "TEST: mov MEM FAC2 const 1.0"
 fill $69 $6E FF FF FF FF FF AA
 r A=$BC
@@ -162,7 +172,7 @@ r A=$E0
 r Y=$B9
 r PC=$BA8C
 ret
-fill $1001 $1006 80 80 00 00 00 FF
+fill $1001 $1006 80 80 00 00 00 80
 print "COMPARE"
 compare $69 $6E $1001
 print "END"
@@ -172,7 +182,7 @@ fill $69 $6E FF FF FF FF FF AA
 fill $22 $23 E0 B9
 r PC=$BA90
 ret
-fill $1001 $1006 80 80 00 00 00 FF
+fill $1001 $1006 80 80 00 00 00 80
 print "COMPARE"
 compare $69 $6E $1001
 print "END"