Tandy 1000 HX Technical Reference Manual page 394

Get Shift Status
Return the current shift status.
Entry Conditions:
AH = 2
Exit Conditions:
AL = current shift status (bit settings: set = true, reset=false)
bit 0 = RIGHT SHIFT key depressed
bit 1 = LEFT SHIFT key depressed
bit 2 = CTRL (control) key depressed
bit 3 = ALT (alternate mode) key depressed
bit 4 = SCROLL state active
bit 5 = NUMBER lock engaged
bit 6 = CAPS lock engaged
bit 7 = INSERT state active