;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;                                                                          ;;
;; This file is part of 'M', a MIPS system emulator.                        ;;
;;                                                                          ;;
;; (C) 2019 Stanislav Datskovskiy ( www.loper-os.org )                      ;;
;; http://wot.deedbot.org/17215D118B7239507FAFED98B98228A001ABFFC7.html     ;;
;;                                                                          ;;
;; You do not have, nor can you ever acquire the right to use, copy or      ;;
;; distribute this software ; Should you use this software for any purpose, ;;
;; or copy and distribute it to anyone or in any manner, you are breaking   ;;
;; the laws of whatever soi-disant jurisdiction, and you promise to         ;;
;; continue doing so for the indefinite future. In any case, please         ;;
;; always : read and understand any software ; verify any PGP signatures    ;;
;; that you use - for any purpose.                                          ;;
;;                                                                          ;;
;; See also http://trilema.com/2015/a-new-software-licensing-paradigm .     ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

section .text

;-----------------------------------------------------------------------------
; Dispatch via rax, given index Register, and given Table (of 32-bit offsets)
;-----------------------------------------------------------------------------
%macro JTABLE   2  ; %1 : index register, %2 : table offset
        mov     eax, dword [%2 + 4 * %1]
        jmp     rax
%endmacro
;-----------------------------------------------------------------------------

;----------------------------
; Start of MIPS CPU Cycle : |
;-----------------------------------------------------------------------------
_wait_cycle:
_cycle:
        inc     CP0_Count            ; Timer: CP0_Count := CP0_Count + 1
        
        ;; Timer Interrupt
        cmp     CP0_Count, CP0_Compare ; Has timer reached limit?
        jne     _cycle_no_mips_timer ; If not, do not invoke interrupt
        SetIRQ  TIMER_IRQ            ; Timer reached limit, invoke timer IRQ

_cycle_no_mips_timer:

        ;; Test if Interrupts are Disabled:
        bt      CP0_Status, CP0St_IE ; CF := CP0St_IE
        jnc     _cycle_no_irq        ; If 0, IRQs are disabled, go to no_irq
        test    CP0_Status, (1 << CP0St_ERL) | (1 << CP0St_EXL) ; If ERL/EXL:
        jnz     _cycle_no_irq        ; ... then also interrupts are disabled
        
        GetSlaveIRQ                  ; See if slave threw interrupt
        
        ;; Interrupts are Enabled; handle any pending Interrupt:
        mov     eax, 0xFC00          ; ( 0xFC00    &
        and     eax, CP0_Cause       ;   CP0_Cause &
        and     eax, CP0_Status      ;   CP0_Status )
        jz      _cycle_no_irq        ; == 0 ? Then no pending IRQ, go to no_irq.

        ;; If here, there is a pending interrupt, service it:
_cycle_irq_handler:

        Flg_Off Waiting              ; Clear Waiting Flag
        SetEXC  EXC_Int              ; Set the EXC_Int Exception Code

        ;; Copy InDelaySlot Flag to RunningDelaySlot Flag:
        Flg_Cpy RunningDelaySlot, InDelaySlot
        
        jmp     _Handle_Exception    ; Handle exception and end cycle.

_cycle_no_irq:                       ; If ints disabled or none pending

        Flg_Get Waiting              ; CF := Waiting Flag
        jc      _wait_cycle          ; If Waiting, start next cycle, else:
        
        ;; Copy InDelaySlot Flag to RunningDelaySlot Flag:
        Flg_Cpy RunningDelaySlot, InDelaySlot
        
        ;; Fetch the instruction to execute:
        mov     eax, PC              ; vAddr := PC
        call    _Virt_Read_Word      ; EAX   := MEM[Virt2Phys(PC)]
        mov     r_I, eax             ; Store fetched instruction
        
        ;; Dispatch the current instruction:
        mov     eax, r_I             ; Get the original instruction :

        ;; 'Accelerated NOPs'
        test    eax, eax             ; Is the current instruction null?
        jnz     _not_nop             ; ... if so, skip dispatch entirely!
        jmp     _end_cycle           ; go to tail of this cycle
_not_nop:                            ; If not a NOP:

        shr     eax, 26              ; Get I-field (upper 6 bits of instr) ;
        JTABLE  eax, _I_Table        ; Dispatch on I-field via I-Table.
        ;;; Instruction executes, after which goes to _end_cycle
;-----------------------------------------------------------------------------

;---------------------------------
; Tail End of a MIPS CPU Cycle : |
;-----------------------------------------------------------------------------
_end_cycle:
        Flg_GOF RunningDelaySlot     ; Were we running the delay slot?
        jnc     _end_cycle_was_not_in_delay_slot ; if not, skip, else:
        ;; We WERE running in the delay slot:
_end_cycle_was_in_delay_slot:        ; If we were running the delay slot:
        mov     PC, nPC              ; PC := nPC
        Flg_Off InDelaySlot          ; Clear the InDelaySlot Flag
        jmp     _cycle               ; Go to next cycle without advancing PC
_end_cycle_was_not_in_delay_slot:    ; If we were NOT running the delay slot:
        add     PC,  0x4             ; PC := PC + 4
        ;; Test for Shutdown Condition:
        Flg_Get Shutdown             ; CF := Shutdown Flag
        jc      _shutdown            ; ... if CF, then shut down.
        jmp     _cycle               ; ... otherwise, go to next cycle.
;-----------------------------------------------------------------------------