Fix: HALF and DOUBLE are signed instructions
Fix: HALF and DOUBLE are signed instructions

I noticed this when the following program would run on the real thing
but not in the simulator:

?mem=65,5,161,34,66,254,45,69

JMP 1
LOAD 5
SHL 1
TSG 2
JMP 2
HALF (6) //doesn't take an argument but needs encoding with 6 as argument for LOAD 5 and TSL 5.
TSL 5
JMP 5

aquila12/c88-js,aquila12/c88-js