; event system routines

const event_stack:         0x01FFFFFC ; pre-decremented
const event_stack_pointer: 0x01FFFFFC

; event types
const mouse_click_event_type:    0x00000000
const menu_bar_click_event_type: 0x00000001
const submenu_update_event_type: 0x00000002
const submenu_click_event_type:  0x00000003
const empty_event_type:          0xFFFFFFFF

; block until an event is available
; inputs:
; none
; outputs:
; r0: event type
; r1-r5: event parameters
wait_for_event:
    ise
    halt

    ; check the event stack pointer
    ; if equal to its base address, then the event stack is empty
    cmp [event_stack_pointer], event_stack_pointer
    ifz jmp wait_for_event

    ; an event is available in the event stack, pop it from the stack and return it
    call pop_event

    ret

; push an event to the event stack
; inputs:
; r0: event type
; r1-r5: event parameters
; outputs:
; none
push_event:
    push r6

    mov r6, [event_stack_pointer]
    ; TODO: check to make sure we don't accidentally clobber the system stack by pushing too many events

    ; push r0
    sub r6, 4
    mov [r6], r0

    ; push r1
    sub r6, 4
    mov [r6], r1

    ; push r2
    sub r6, 4
    mov [r6], r2

    ; push r3
    sub r6, 4
    mov [r6], r3

    ; push r4
    sub r6, 4
    mov [r6], r4

    ; push r5
    sub r6, 4
    mov [r6], r5

    mov [event_stack_pointer], r6

    pop r6
    ret

; pop an event from the event stack
; inputs:
; none
; outputs:
; r0: event type
; r1-r5: event parameters
pop_event:
    ; check the event stack pointer
    ; if equal to its base address, then the event stack is empty
    cmp [event_stack_pointer], event_stack_pointer
    ifz jmp pop_event_empty

    push r6

    mov r6, [event_stack_pointer]

    ; pop r5
    mov r5, [r6]
    add r6, 4

    ; pop r4
    mov r4, [r6]
    add r6, 4

    ; pop r3
    mov r3, [r6]
    add r6, 4

    ; pop r2
    mov r2, [r6]
    add r6, 4

    ; pop r1
    mov r1, [r6]
    add r6, 4

    ; pop r0
    mov r0, [r6]
    add r6, 4

    mov [event_stack_pointer], r6

    pop r6
    ret
pop_event_empty:
    mov r0, empty_event_type
    mov r1, 0
    mov r2, 0
    mov r3, 0
    mov r4, 0
    mov r5, 0

    ret