# Intel 8259A: mysterious T bit doesn't fit the pattern

I'm implementing a driver for the Intel 8259A PIC for x86. To do this properly, I am reading the Intel 8259A PIC Datasheet. It is halfway comprehensible even for someone who approaches controller chips exclusively from the software perspective like me but there is this diagram I don't get:

It is the 8-bit pointer sent from the controller to the CPU in response to the second #INTA (interrupt acknowledgement) raise.
IR is an abbreviation for "interrupt request," the D bits denote the data lines of the 8-bit data buffer of the controller.

What are these T3, T4, ..., T7? The datasheet does not mention them ever again, they are just there. I already wrote a driver for the Intel 8042 (guided by the IBM 5170 (PC AT) Technical Reference) and the T0 and T1 bits were called "test inputs" there (p. 1-44).

Is that related to the T bits from the description of the Intel 8042 controller? What other meaning could these names have?

• Those bits come from ICW2. See the bottom of Fig.7 on pg.11 – brhans Mar 28 '16 at 20:23
• @brhans Oh, well-spotted, thanks! So the T bits are initialized with ICW2 and set according to that ICW in future vector bytes (the said 8-bit pointer). Is this possible? – cadaniluk Mar 28 '16 at 20:28
• That IBM PC AT technical reference is a gem! Thanks for including the link. – Jet Blue Jun 22 '19 at 6:10