;--------------------
; testINT.asm
; Test interrupt mechanisms.
; Load this at x0200
; LC3 hardware revision: INTs are off at power up.
;
; Aside: LC3 starts in super mode (PSR[15] = 0). Can switch
; to user mode by executing RTI with preset PSR word on stack.
; Then use illegal opcode exception to switch to super mode.
; This can be used in place of the LC3 trap mechanism as a way
; to jump into OS code from user mode.
;--------------------
.ORIG x0200

    ;------- Initialize keyboard interrupt handling.

    ;-- Set up super's stack.
    ld r6, stackPtr ;--    r6 <== pointer to bottom of stack.

    ;-- Set up IVT for keyboard interrupt.
    ld r0, kbIVTloc ;--    r0 <== addr of kb's vector.
    lea r1, kbCode  ;--    r1 <== kbCode's address.
        ...         ;--    MEM[vec addr] <== kbCode's address.

    ;-- Enable KB INTs, wait for an interrupt.
    ldi r3, KBSR    ;--    r3 <== current content of KBSR.
    ld r4, ENmask   ;--    r4 <== interrupt enable mask.
        ...         ;--    ...
    sti r3, KBSR    ;--    KBSR <== r3

    ;-- Global INTs on. (see Modules/ints.asm)
        ...             ;-- new_PSR_value <== PSR_INTS_ON
    push__( R0 )        ;-- push new_PSR_value
        ....            ;-- fake_PC <== HERE
    push__( R0 )        ;-- push fake_PC
    rti                 ;-- PC <== HERE, PSR <== PSR_INTS_ON

    ;------- Faking execution of user's or super's program:
    HERE:
    ld r3, Limit      ;-- cnt <== Limit
    LOOP:             ;-- Loop
        mov__(r3,r3)  ;--    test cnt
    BRp LOOP          ;-- Until(cnt <= 0)
    ld  r4, STOP      ;-- r4 <== Disable sys_clk signal.
    sti r4, MCR       ;-- MCR <== r4: shuts down hardware.

  ;------- Keyboard interrupt handler. NB--Has side effects!
  KBcode:
    add r3, r3, -1    ;-- do something on INT: cnt--
    ldi r0, KBDR      ;-- get data and handshake kb_ctl (KBSR[15]<==0)
    rti

 ;------- Data area.
  kbIVTloc:      .FILL x0180  ;-- keyboard's vector slot in IVT.
  stackPtr:      .FILL x3000  ;-- address of bottom of stack.
  ENmask:        .FILL x4000  ;-- ENmask[14]=1 enable INTs
  Limit:         .FILL x0003  ;-- Allow for 3 interrupts
  KBSR:          .FILL xFE00  ;-- address of KBSR.
  KBDR:          .FILL xFE02  ;-- address of KBDR.
  PSR_INTS_ON:   .FILL x0000  ;-- PSR={Priv=0, Prio=7, CC=0}
  MCR:           .FILL xFFFE  ;-- address of mach. ctl reg
  STOP:          .FILL x8000  ;-- MCR[15]=1 disables sys_clk

.END