Hello,
I'm having some issues with KnightOS.  As of right now, it runs fine on the TI-83+ (SE).  However, on the TI-84+ (SE), it crashes.  I believe this to be caused by improper USB interrupt handling.  KnightOS runs without incident if interrupts are turned on, but as soon as interrupts (and multitasking) are turned on, it will crash.  Here is my USB interrupt code:
HandleUSBEvents:
ld a, (ModelID)
cp 2 ; Models 2 and up have USB.
jp c, ResumeInterrupt ; Leave if we don't have the hardware (also tested with NC)
in a, (55h) ; Read USB status
bit 0, a
jr z, UnknownUSBEvent
bit 2, a
jr z, USBLineInterrupt
bit 4, a
jp z, USBProtocolInterrupt
jp ResumeInterrupt

UnknownUSBEvent:
xor a
out (5Bh), a ; Unknown event, WikiTI claims you should zero port 5B to acknowledge
jp ResumeInterrupt

USBLineInterrupt:
in a, (56h)
ld c, a
cpl ; xor 0FFh
ld b, a
in a, (57h)
ld d, a
and b
out (57h), a
ld a, d
out (57h), a ; Ack the interrupt without doing anything special
jp ResumeInterrupt

USBProtocolInterrupt:
in a, (86h)
ld c, a
in a, (84h)
ld b, a
in a, (85h)
in a, (83h)
in a, (82h) ; Just ack and leave, and hope that it doesn't get angry

jp ResumeInterrupt
However, if you don't notice anything wrong with this, it may not be the problem.  Also suspect is the code that sets up the memory mappings on boot:
    ld a, 1    ; Set flash page 1 in bank 1.
    out (6), a
    in a, (2)    ;get calc version
    rlca ;Roll bit 7 into carry.
    ld a, 41h ; Set ram page 1 in bank 2.
    jr nc, LowerModel ; C vs NC?
HigherModel:
    out (5),a
    ld a,81h
    out (7),a
    jr Return
LowerModel:
    out (6), a
    out (7),a
Return:
    retI'd put my money on USB interrupts, but that's just me.  You are now looking at the only model-dependent code in the whole OS, so this is pretty much where the problem has to lie.  Is there any hardware I may not have initialized properly, or something I may have left out that is required on the TI-84+ (SE)?

Ah, that could present an issue.  I want an executable page in both, though, is that possible?  Compatibility with calculators missing RAM is vital.

Yeah, you can put an odd-numbered page in (usually people use 3, but that's due to Omnicalc using pages 4-7, which you don't have to worry about for KOS)

If I understand correctly you just need to ld a with $83 rather than $81

No, that's the $8000-$BFFF region. What you need to do is output 3 to port 5 instead of $40

Edit:
Instead of $41, I meant >_>

For the "higher models"
However, you made an optimization of reusing a which means you'll have to ld a another time

Good luck Sir! Hope it works!

For the record, this part of the code doesn't make sense:
ld a, (ModelID)
cp 2 ; Models 2 and up have USB.
jp c, ResumeInterrupt ; Leave if we don't have the hardware (also tested with NC)
This code doesn't make sense in my brain. Say the ModelID is 4. cp 2 would set the carry flag, right? So jp c would jump back for all the models with USB and try and run on those without it.

For that protocol interrupt, if it wants to send you some data over the control pipe, it might not like you ignoring it.  For now, I think you might be able to just stall the ports and that will prevent it from connecting at all, but it should keep the driver working.