diff options
Diffstat (limited to 'interrupts.asm')
| -rw-r--r-- | interrupts.asm | 105 |
1 files changed, 105 insertions, 0 deletions
diff --git a/interrupts.asm b/interrupts.asm index f76a0ff..78ff87d 100644 --- a/interrupts.asm +++ b/interrupts.asm @@ -1,4 +1,109 @@ +;;; interrupt handling code + + ;; Current interrupt mode. IM 1 and friends will modify + ;; this. + + ;; IM 0: A byte is placed on the bus and executed as if it + ;; were inline in the program. This emulator will put that + ;; byte into int_opcode and set epc (or int_jump) to point + ;; there. + + ;; IM 1: RST 38 is executed on every interrupt. + + ;; IM 2: Vectored, the address jumped to is as follows: + ;; + ;; (I << 8) | (byte & 0xfe) + ;; + ;; where I is the I register, and byte is the byte that was + ;; found on the bus. +int_mode: dc.b 0 + + ;; 0 if interrupts are turned on. + ;; 1 if they are being held. +int_held: dc.b 0 + + ;; 0 if no interrupt is pending. + ;; 1 if an interrupt occurred while interrupts were being + ;; held. + ;; 3 (==1|2) if an NMI is pending. +int_waiting: dc.b 0 + + ;; The value of epc as a result of a held interrupt. This is + ;; stored as a derefenced value (pointer into host memory). + + ;; I store it as a native pointer because Z80 interrupts can + ;; be very strange. Interrupt mode 0 in particular requires + ;; a shim. +int_jump: dc.l 0 + + ;; In interrupt mode 0, an interrupt will force a byte onto + ;; the data bus for the processor to execute. To handle + ;; those, the emulator will store the bus byte in int_opcode, + ;; which is followed by an absolute jump to the "next + ;; instruction". The value of int_jump will then be + ;; &int_opcode. +int_opcode: dc.b 0 + dc.b $c3 ; JP immed.w +int_return: dc.w 0 ; the destination address + + + ;; This is a macro to hold interrupts. + + ;; When interrupts are "disabled" (held), host interrupts will + ;; still fire. The ISR will see that they are held and update + ;; the above fields with the interrupt type and location. + ;; Then the EI macro to enable them will cause it to fire. +HOLD_INTS MACRO + moveq.b #1,int_held ; 4 cycles + ENDM + + ;; This is a macro to release a held interrupts. +CONTINUE_INTS MACRO + bsr ints_continue ; 18 cycles + ENDM + +ints_continue: + tst.b int_waiting ; 4 cycles + bne.b ints_continue_pending ; 8 cycles not taken + ;; Common case: no interrupt pending + moveq.b #0,int_held ; 4 cycles + rts ; 16 cycles + ;; typical case: 4+18+4+8+4+16 = 54 cycles + + ;; I can go faster (24 cycles typical case) by using 68k + ;; hardware interrupt disable/reenable. +ints_continue_pending: + subq.b #3,int_waiting + beq int_do_nmi + move.b int_mode, + + + ;; This routine emulates a mode 0 interrupt. +int_do_mode0: + rts + + ;; This routine emulates a mode 1 interrupt. +int_do_mode1: + rts + + ;; This routine emulates a mode 2 interrupt. +int_do_mode2: + rts + + ;; This routine emulates a non-maskable interrupt. +int_do_nmi: + rts + + + + + ;; This routine is used by the emulated DI instruction, which + ;; turns off emulator interrupts. ints_stop: rts + + ;; This routine is used by the emulated EI instruction, which + ;; turns on emulator interrupts. ints_start: rts + |