@let SP              RH

; vector table fields
@let vector_table    0x10
@let vt_ISR0         0x10 ; ISR0
@let vt_ISR1         0x11 ; ISR1
@let vt_ISR2         0x12 ; ISR2
@let vt_ISR3         0x13 ; ISR3
@let vt_stack_addr   0x14 ; Stack address
@let vt_term_pp_addr 0x15 ; Terminal peripheral address
@let vt_kb_pp_addr   0x16 ; Keyboard peripheral address
@let vt_gr_mode_addr 0x17 ; Graphics mode setting address
@let vt_sprite_mem   0x18 ; Sprite memory address
@let vt_text_mem     0x19 ; Text memory buffer address

; memory segments
@let stack_segment   0x8000
@let mmio_segment    0xE000
@let terminal_addr   0xE000
@let keyboard_addr   0xE001
@let gr_mode_addr    0xE002
@let sprite_mem      0xE800
@let text_mem        0xF800 ; to 0xFFFF
; note: sprite memory & text memory can use the same space
; since they are never used at the same time

; graphics mode settings
@let gr_disabled_mode 0b00
@let gr_text_mode     0b01
@let gr_sprite_mode   0b10

@address vector_table
  keyboard_isr   ; 0x10
  hlt_isr        ; 0x11
  hlt_isr        ; 0x12
  hlt_isr        ; 0x13
  stack_segment  ; 0x14
  terminal_addr  ; 0x15
  keyboard_addr  ; 0x16
  gr_mode_addr   ; 0x17
  sprite_mem     ; 0x18
  text_mem       ; 0x19

@label hlt_isr
  ldi 0x55 RE
  hlt

@label keyboard_isr
  spu RA
  spu RB
  ldi vt_kb_pp_addr RA    ; RA := Keyboard peripheral address pointer
  ldr RA RA               ; RA := Keyboard peripheral address deref
  ldr RA RA               ; RA := <16-bit keyboard character code> from MMIO register
  ldi vt_term_pp_addr RB  ; RB := Terminal peripheral address pointer
  ldr RB RB               ; RB := Terminal peripheral address deref
  str RA RB               ; terminal <- character code (ASCII?)
  mov RA RE               ; RE := character code (for debugging)
  spo RB
  spo RA
  rti
@label program_segment

; define short prelude that sets up vital instructions
; NOTE: must be shorter than the vector table offset
@address 0x0
; set up stack pointer to point to beginning of stack segment
  ldi vt_stack_addr RA
  ldr RA SP
  jpi program_segment

@address program_segment
; set graphics mode to disabled
  ldi vt_gr_mode_addr RA
  ldr RA RA
  ldi gr_disabled_mode RB
  str RB RA
; jump to main (user defined)
  jpi main