sft may be implemented incorrectly

[chdft]

The official specification is not clear on the interpretation of the right register argument.
It is currently interpreted as signed value, since that enables right shifts. Any hints reference documentation on this topic are appreciated.

[chdft]

The test program

mov $0 16
mov $3 2
mov $4 -2
sft $1 $0 $3
sft $2 $0 $4

results in the following register values using the official simulator:

00: 00.10
01: 00.40
02: 00.00
03: 00.02
04: FF.FE

This indicates, that the right parameter of sft is treated as unsigned value (shifting 16 by 65534 bits), resulting in an empty (=> 0) value for register 2.

Currently, AbaSim treats the right parameter as signed to enable right shifts, which are otherwise cumbersome.