NewIRQs

; Copyright 1996 Acorn Computers Ltd
;
; Licensed under the Apache License, Version 2.0 (the "License");
; you may not use this file except in compliance with the License.
; You may obtain a copy of the License at
;
;     http://www.apache.org/licenses/LICENSE-2.0
;
; Unless required by applicable law or agreed to in writing, software
; distributed under the License is distributed on an "AS IS" BASIS,
; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
; See the License for the specific language governing permissions and
; limitations under the License.
;
        TTL     => NewIRQs

; *****************************************************************************
;
; Main IRQ routine:
;     Push  workregs,lr
;     IRQsema -> TOS
;     stack -> IRQsema
;     call IRQ1V
;     IRQs off
;     TOS ->IRQsema
;     process callback, pulling workregs,pc at some point
;
; *****************************************************************************

        ALIGN 32

Initial_IRQ_Code ROUT
        SUB     lr, lr, #4
        Push    "r0, lr"
        MRS     lr, SPSR
        Push    "r1-r3, r11, r12, lr"
; ** For Pete's sake remember to change the heap manager if you change the above
; ** register list!!!!!!! And the [sp_irq, #4*5] below

  [ :LNOT:No26bitCode
        BIC     r0, lr, #&1F            ; clear out foreground mode bits
        ORR     r0, r0, #I32_bit + IRQ26_mode ; force IRQ_26 mode and I bit set
        MSR     CPSR_c, r0
  ]

        LDR     r12, =ZeroPage
        LDR     r0, [r12, #IRQsema]
        Push    r0
        STR     sp_irq, [r12, #IRQsema]
        MOV     lr, pc
        LDR     pc, [r12, #IRQ1V]

; IRQ1V called with r0-r3,r11,r12 trashable. r12=0

; Stu has a theory that 1N cycle can be saved by the default IRQ1V pointing
; at a location containing a branch to our code; we then do something like
;  LDR R0, [R12, #IRQ1V]
;  CMP R0, #OurIRQ1V
;  BNE somebodysonIRQ1V
;  .... fall into default IRQ1V code

        Push     "r10"
        MOV      r10, #UnthreadV
        BL       CallVector
        Pull     "r10"

        LDR      r11, =ZeroPage
        Pull     r0
        STR      r0, [r11, #IRQsema]

  [ :LNOT:No26bitCode
        MRS      r0, CPSR
        ORR      r0, r0, #&10
        MSR      CPSR_c, r0                     ; switch back to IRQ32 mode
  ]

        LDRB     r11, [r11, #CallBack_Flag]
        TEQ      r11, #0
        Pull     "r1-r3, r11, r12, lr", EQ
        MSREQ    SPSR_cxsf, lr
        Pull     "r0, pc", EQ, ^

        TST      r11, #CBack_Postpone
        LDREQ    lr, [sp_irq, #4*5]     ; get SPSR off stack
        TSTEQ    lr, #I32_bit :OR: &0F  ; check we came from USR26 or USR32 mode, with IRQs enabled
        Pull     "r1-r3, r11, r12, lr", NE
        MSRNE    SPSR_cxsf, lr
        Pull     "r0, pc", NE, ^

; Do a CallBack: asked for, not postponed, and we're returning into USR26/32 mode.

        ASSERT   IRQ32_mode :AND: SVC32_mode = IRQ32_mode ; so the following dodgy ops work

        Pull     "r1-r3, r11, r12"
        MRS      r0, CPSR
        ORR      r0, r0, #SVC32_mode
        MSR      CPSR_c, r0
        Push     "r10-r12"                               ; push r10-r12 onto the SVC stack
        BIC      r0, r0, #IRQ32_mode :EOR: SVC32_mode
        MSR      CPSR_c, r0
        Pull     "r10-r12"                      ; SPSR, R0, LR really
 [ No26bitCode :LOR: FixCallBacks
        ORR      r0, r0, #SVC32_mode
 |
        BIC      r0, r0, #&1F
        ORR      r0, r0, #SVC26_mode
 ]
        MSR      CPSR_c, r0
        Push     r12                            ; Save the return address
        MOV      r14, r10                       ; SPSR into R14
        MOV      r0, r11                        ; restore original R0
        LDR      r10, =ZeroPage
        LDRB     r11, [r10, #CallBack_Flag]
      [ FixCallBacks
        B        Do_CallBack_postpone_already_clear
      |
        B        Do_CallBack
      ]

        LTORG

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Default IRQ1V: despatch on interrupting device

; Now copied to RAM, together with vector entries and device tables

        ASSERT    HAL ; Pre-HAL code is gone. Sorry!

                  ^ 0
IRQDesp_Link      # 4
IRQDesp_R12Val    # 4
IRQDesp_CallAddr  # 4

IRQDesp_Link_Unshared * 1       ; flag in Link (for _this_ node)

       ASSERT   IRQDesp_CallAddr = IRQDesp_R12Val + 4

DefaultIRQ1Vcode ROUT

        Push    "r9,lr"
      [ ZeroPage = 0
        MOV     r9, #0
      |
        LDR     r9, %FT02
      ]
        AddressHAL r9                   ; modifies r9
        CallHAL HAL_IRQSource
        Pull    "r9"
        ADR     r2, Devices
        ADD     r1, r0, r0, LSL #1      ; multiply by 3
        ADD     r11, r2, r1, LSL #2     ; so table contains DevNo * 3
01      MOV     lr, pc
        LDMIA   r11, {r11, r12, pc}
        TST     r11, #IRQDesp_Link_Unshared
        BEQ     %BT01
        Pull    pc
      [ ZeroPage <> 0
02
        DCD     ZeroPage
      ]

; ******* IRQ device handlers entered with r0-r3,r11,r12,r14 trashable *******
;   r0  =  device number (if HAL)
;   r3  -> IOC (not in HAL world, unless IOMD HAL is helpful)
;   r12 =  what they asked for
;   r14 =  return address to MOS IRQ exit sequence

DefaultIRQ1Vcode_end

Devices * DefaultIRQ1Vcode_end + 12

NoInterrupt     * -1

DevicesEnd * Devices + MaxInterrupts * 12

        ASSERT  DevicesEnd - DefaultIRQ1Vcode <= DefIRQ1Vspace

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

InitialiseIRQ1Vtable

        Push    "v1-v3,sb,lr"
; copy IRQ handler: not done with rest of copying
; because soft break needs the info to free any claimed blocks.

        LDR     a1, =DefaultIRQ1V
        ADRL    a2, DefaultIRQ1Vcode
        ADRL    a3, DefaultIRQ1Vcode_end
CopyDefaultIRQ1V
        LDR     a4, [a2], #4
        STR     a4, [a1], #4
        CMP     a2, a3
        BNE     CopyDefaultIRQ1V

        AddressHAL
        ADD     v1, a1, #12
        ADD     a3, v1, #MaxInterrupts*12
        MOV     a2, #0
        MOV     a4, #-1
        LDR     ip, =IRQ
FillInDefaultIRQ1VDevices
        STMIA   a1!, {a2, a4, ip}
        ADD     a4, a4, #1
        CMP     a1, a3
        BNE     FillInDefaultIRQ1VDevices

; Now fill in our basic device handlers. First the timer.
        MOV     a1, #0
        CallHAL HAL_TimerDevice
        LDR     a2, =ZeroPage+OsbyteVars
        LDR     a3, =TickOne
        ADD     a1, a1, a1, LSL #1
        ADD     a1, v1, a1, LSL #2
        STMIB   a1, {a2, a3}

1
 [ NumberOfPodules > 0
; Now Podule bits
        MOV     a1, #IRQDesp_Link_Unshared
        LDR     a2, =ZeroPage+PFIQasIRQ_Chain - (PodDesp_Link-PodDesp_R12Val)
        ADR     a3, PFIQasIRQ_Despatch
        ADD     lr, v1, #PFIQasIRQ_DevNo*12
        STMIA   lr, {a1, a2, a3}
        LDR     a2, =ZeroPage+PIRQ_Chain - (PodDesp_Link-PodDesp_R12Val)
        ADR     a3, PIRQ_Despatch
        ADD     lr, v1, #Podule_DevNo*12
        STMIA   lr, {a1, a2, a3}
 ]

; Now IIC - if any
        LDR     v2, =ZeroPage+IICBus_Base
        MOV     v3, #0
80
        LDR     a1, [v2, #IICBus_Type]
        TST     a1, #IICFlag_HighLevel
        TSTNE   a1, #IICFlag_Background
        BEQ     %FT90
        MOV     a1, v3
        CallHAL HAL_IICDevice
; I think it's safe to call A SWI here...
        LDR     r1, =IICIRQ
        MOV     r2, v3
        SWI     XOS_ClaimDeviceVector
90
        ADD     v2, v2, #IICBus_Size
        ADD     v3, v3, #1
        CMP     v3, #IICBus_Count
        BNE     %BT80

        Pull    "v1-v3,sb,pc"


; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Specialist despatchers for podules

                  ^  0
PodDesp_Address   #  4     ; address of IRQ status byte
PodDesp_Mask      #  4     ; for use on above
PodDesp_R12Val    #  4
PodDesp_CallAddr  #  4     ; address to call if (?Address EOR (Mask>>8)) AND Mask <> 0
PodDesp_Link      #  4     ; next node
PodDesp_NodeSize  #  0

; In    r12 =    PFIQasIRQ_Chain - (PodDesp_Link-PodDesp_R12Val)
;             or PIRQ_Chain - (PodDesp_Link-PodDesp_R12Val)     from despatcher

PFIQasIRQ_Despatch ROUT
PIRQ_Despatch ; All the same thing now

01      LDR     r12, [r12, #PodDesp_Link-PodDesp_R12Val]
        LDMIA   r12!, {r1, r2}           ; address and mask

; TMD 09-Jun-89: Don't corrupt r0 - it's needed by the default IRQ2 routine
        LDRB    r1, [r1]
        EOR     r1, r1, r2, LSR #8
        ANDS    r1, r1, r2
        BEQ     %BT01
        LDMIA   r12, {r12, pc}

Default_PIRQHandler_Node
Default_PFIQasIRQHandler_Node
        &       .+4                     ; address we know has non-zero value!
        &       -1                      ; mask
        &       0                       ; handler r12
        &       IRQ                     ; handler code
        &       0                       ; null link for naff release checking

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Claim of device vectors

; r0 = Device number
; r1 = call address
; r2 = r12 value
; r3 = interrupt location      } when podules are
; r4 = interrupt mask/polarity } supported and r0 = podule device number

CDV_Flags * &FF000000
CDV_Shared * 1:SHL:31

DeviceVector_Claim ROUT

        Push    "r0-r3, lr"

01      SWI     XOS_ReleaseDeviceVector ; Release until bored
        BVC     %BT01

        LDR     r0, [sp]
 [ HAL
        BIC     r0, r0, #CDV_Flags
 ]
        LDR     lr, =ZeroPage
        LDR     lr, [lr, #IRQMax]
        CMP     r0, lr
        BHS     DV_Fail_NaffDevNo

 [ NumberOfPodules > 0
        ! 0,    "ClaimDeviceVector has podule IRQ/FIQs assembled in"
        CMP     r0, #Podule_DevNo
        CMPNE   r0, #PFIQasIRQ_DevNo
        BEQ     PoduleChainClaim
 ]
        MOV     r3, #12
        BL      ClaimSysHeapNode
        BVS     DV_Exit
        LDR     r11, [sp]
        BIC     r0, r11, #CDV_Flags
        ADD     r0, r0, r0, LSL #1      ; *3
        LDR     r1, =DefaultIRQ1V-DefaultIRQ1Vcode+Devices
        ADD     r1, r1, r0, LSL #2
        WritePSRc SVC_mode+I_bit, r10   ; IRQs off for update (on again on SWI exit)
        LDMIA   r1, {r0, r3, r10}
        STMIA   r2, {r0, r3, r10}       ; copy current head into node
 [ HAL
        MOV     r10, r2
        TST     r11, #CDV_Shared
        ORREQ   r10, r10, #IRQDesp_Link_Unshared
        LDR     r11, [sp, #4*2]         ; r12 value
        LDR     r12, [sp, #4*1]         ; call address
 |
        LDR     r10, [sp, #4*2]         ; r12 value
        LDR     r11, [sp, #4*1]         ; call address
        MOV     r12, r2
 ]
        STMIA   r1, {r10-r12}           ; copy given info into head


DV_Exit STRVS   r0, [sp]                ; Common exit for both claim + release
        Pull    "r0-r3, lr"
        B       SLVK_TestV


DV_Fail_NaffDevNo
        ADR     r0, ErrorBlock_NaffDevNo
      [ International
        BL      TranslateError
      |
        SETV
      ]
        B       DV_Exit

        MakeErrorBlock NaffDevNo

 [ NumberOfPodules > 0
PoduleChainClaim
        MOV     r3, #PodDesp_NodeSize
        BL      ClaimSysHeapNode
        BVS     DV_Exit
        MOV     r10, r2
        LDMFD   sp, {r0-r3}
        STR     r1, [r10, #PodDesp_CallAddr]
        STR     r2, [r10, #PodDesp_R12Val]
        STR     r3, [r10, #PodDesp_Address]
        STR     r4, [r10, #PodDesp_Mask]
        CMP     r0, #Podule_DevNo
        LDREQ   r0, =ZeroPage+PIRQ_Chain
        LDRNE   r0, =ZeroPage+PFIQasIRQ_Chain
        WritePSRc SVC_mode+I_bit, r1    ; IRQs off for update
        LDR     r1, [r0]
        STR     r1, [r10, #PodDesp_Link]
        STR     r10, [r0]
        B       DV_Exit
 ]

; .............................................................................
; Release of device vectors

; r0 = Device number
; r1 = call address
; r2 = r12 value
; r0 = PFIQ|PIRQ devno -> r3 = interrupt location (LDRB always used)
;                         r4 = interrupt mask

DeviceVector_Release ROUT

        Push    "r0-r3, lr"             ; Ensure same regset as above
 [ HAL
        BIC     r0, r0, #CDV_Flags
 ]
        LDR     lr, =ZeroPage
        LDR     lr, [lr, #IRQMax]
        CMP     r0, lr
        BHS     DV_Fail_NaffDevNo

        WritePSRc SVC_mode + I_bit, r12 ; IRQs off while holding context
 [ NumberOfPodules > 0
        ! 0,    "ReleaseDeviceVector has podule IRQ/FIQs assembled in"
        CMP     r0, #Podule_DevNo
        CMPNE   r0, #PFIQasIRQ_DevNo
        BEQ     PoduleChainRelease
 ]

        ADD     r0, r0, r0, LSL #1      ; *3
        LDR     r12, =DefaultIRQ1V-DefaultIRQ1Vcode+Devices
        ADD     r12, r12, r0, LSL #2    ; address of node
        MOV     r11, #-1                ; "fudge" predecessor node

 [ HAL
01      LDMIB   r12, {r3, r10}
 |
01      LDMIA   r12, {r3, r10}
 ]
        CMP     r3, r2
        CMPEQ   r10, r1
        BEQ     %FT02                   ; found it
        MOV     r11, r12
        LDR     r12, [r12, #IRQDesp_Link]
 [ HAL
        BICS    r12, r12, #IRQDesp_Link_Unshared
 |
        CMP     r12, #0
 ]
        BNE     %BT01

11      ADR     r0, ErrorBlock_BadDevVecRel
      [ International
        BL      TranslateError
      |
        SETV
      ]
        B       DV_Exit

        MakeErrorBlock BadDevVecRel


02      CMP     r11, #-1
        BEQ     %FT03
        MOV     r2, r12
        LDR     r12, [r2, #IRQDesp_Link]
 [ HAL
        LDR     r14, [r11, #IRQDesp_Link]       ; preserve r11's "unshared" flag
        BIC     r12, r12, #IRQDesp_Link_Unshared
        AND     r14, r14, #IRQDesp_Link_Unshared
        ORR     r12, r12, r14
 ]
        STR     r12, [r11, #IRQDesp_Link] ; node delinked
        B       %FT04

03      LDR     r2, [r12, #IRQDesp_Link]; freeable = nextnode
 [ HAL
        BIC     r2, r2, #IRQDesp_Link_Unshared
 ]
        LDMIA   r2,  {r0, r1, r3}       ; copy next node into head posn
        STMIA   r12, {r0, r1, r3}

04
        BL      FreeSysHeapNode         ; free block
        B       DV_Exit

 [ NumberOfPodules > 0
PoduleChainRelease
        CMP     r0, #Podule_DevNo
        LDREQ   r0, =ZeroPage+PIRQ_Chain-PodDesp_Link
        LDRNE   r0, =ZeroPage+PFIQasIRQ_Chain-PodDesp_Link

10      LDR     r12, [r0, #PodDesp_Link]
        CMP     r12, #0
        BEQ     %BT11
        LDR     r11, [r12, #PodDesp_Address]
        CMP     r11, r3
        LDREQ   r11, [r12, #PodDesp_Mask]
        CMPEQ   r11, r4
        LDREQ   r11, [r12, #PodDesp_CallAddr]
        CMPEQ   r11, r1
        LDREQ   r11, [r12, #PodDesp_R12Val]
        CMPEQ   r11, r2
        MOVNE   r0, r12
        BNE     %BT10

        LDR     r11, [r12, #PodDesp_Link]
        STR     r11, [r0,  #PodDesp_Link]
        MOV     r2, r12
        B       %BT04
 ]

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Default device owner for IRQ not recognised by system: pass to IRQ2V

IRQ ROUT

        Push    "r10, lr"
        MOV     r10, #IrqV
        BL      CallVector

 [ HAL
        Pull    "r10, lr"
        Pull    "pc"                    ; new-style CDV - pull return address
 |
        Pull    "r10, pc"               ; return: someone will always claim it.
 ]

; *****************************************************************************
; Default IRQ2V:
 [ HAL
;   r0  must still have devno in it
;   r12 is 0 (from vector)
 |
;   r0  must still have devno*3 in it
;   r12 is 0 (from vector)
 ]

; Clear mask, clear IRQ as appropriate/possible

; NB. a cheap way of dividing by ~3 is *11,/32: accurate for 0..31 result ...

NOIRQ ROUT

 [ HAL
        TEQ     r0, #0
        Pull    pc, MI
        MOV     r11, r9
        AddressHAL
        CallHAL HAL_IRQDisable
        MOV     r9, r11
 |
01      SUBS    r0, r0, #3
        ADDGE   r12, r12, #1
        BGT     %BT01                   ; r12 := r0 DIV 3

        CMP     R12, #8
        MOVLO   R0, #IOCIRQMSKA
        BLO     %FT03

        CMP     R12, #16
        SUBLO   R12, R12, #8
        MOVLO   R0, #IOCIRQMSKB
        BLO     %FT03

        CMP     R12, #IOMD_MouseRxFull_DevNo
        SUBLO   R12, R12, #IOMD_DMAChannel0_DevNo
        MOVLO   R0, #IOMD_DMAMSK
        BLO     %FT03

 [ MorrisSupport
        CMP     R12, #IOMD_C_Bit0_DevNo
        SUBLO   R12, R12, #IOMD_MouseRxFull_DevNo       ;reduce to bit number 0..7
        MOVLO   R0, #IOMD_IRQMSKD                       ; in IRQ D interrupt register

        SUBHS   R12, R12, #IOMD_C_Bit0_DevNo
        MOVHS   R0, #IOMD_IRQMSKC
 ]

03
        ADD     r0, r0, #IOC
        MOV     r1, #1
        MOV     r1, r1, LSL r12         ; bit to clear

        MRS     lr, CPSR
        BIC     r12, lr, #&0F
        ORR     r12, r12, #I32_bit+F32_bit+IRQ_mode
        MSR     CPSR_c, r12
        LDRB    r12, [r0]       ; FIQs off for updating IOCIRQMSKA
        BIC     r12, r12, r1
        STRB    r12, [r0]       ; relevant IRQ disabled
        MSR     CPSR_c, lr      ; absolute minimum FIQ disable period

        STRB    r1, [r0, #IOCIRQCLRA-IOCIRQMSKA] ; Clear IRQ
 ]
        Pull    pc              ; claim vector

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; The following bits have been appropriated from source.pmf.oseven to make
; sure Tim's old code doesn't overwrite us when he gets back!

; SWI OS_GenerateEvent: call event vector if enabled

GenEvent ROUT

        Push    lr
        WritePSRc SVC_mode+I_bit, lr    ; Disable IRQs. MUST call these ones
        BL      OSEVEN                  ; in SVC mode as people expect it
        Pull    lr
        B       SLVK

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Subroutine call version

; In    r0 = event type
;       r1,r2 parameters

; Out   C=0 => event was enabled, or was >= 32 anyway
;       C=1 => event was disabled, so vector not called

OSEVEN ROUT

        Push    lr

        CMP     r0, #31                 ; Events >= 32 are ALWAYS raised. SKS
                                        ; flags are HI if so, ie. NE
      [ ZeroPage = 0
        LDRLSB  r14, [r0, #OsbyteVars + :INDEX: EventSemaphores]
      |
        LDRLS   r14, =ZeroPage+OsbyteVars+:INDEX:EventSemaphores
        LDRLSB  r14, [r0, r14]
      ]
                                        ; get semaphore for this event 0..31
        CMPLS   r14, #0                 ; non-zero => enabled
        Pull    pc, EQ                  ; if disabled, exit with C=1

        Push    "r0-r3, r10-r12" ; r3 excessive ???
        MOV     r10, #EventV            ; call event vector
        BL      CallVector
        CLC                             ; indicate event enabled
        Pull    "r0-r3, r10-r12, pc"

; ...................... default owner of EventV ..............................
; Call Event handler

; In    r12 = EvtHan_ws

DefEvent ROUT

        MOV     lr, pc                  ; link with all the bits
        LDMIA   r12, {r12, pc}          ; call EventHandler, returns to ...

        TEQ     r12, #1
        Pull    pc,NE

      [ ZeroPage = 0
        LDRB    r14, [r12, #CallBack_Flag-1] ; IRQs are still disabled
        ORR     r14, r14, #CBack_OldStyle
        STRB    r14, [r12, #CallBack_Flag-1]
      |
        LDR     r12, =ZeroPage
        LDRB    r14, [r12, #CallBack_Flag] ; IRQs are still disabled
        ORR     r14, r14, #CBack_OldStyle
        STRB    r14, [r12, #CallBack_Flag]
      ]

        Pull    pc                      ; claim EventV

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Process timer zero IRQ device (100Hz clock)
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

        ALIGN 32

TickOne ROUT

 [ HAL
        ; Don't push r14 - we're using new interface, and claim the vector
        Push    "r0,r9,r12"
        AddressHAL

        ; In some chips, Timer0 is latched in the timer block instead of,
        ; or as well as in the interrupt controller
        MOV     R0, #0 ; clear latch for Timer0
        CallHAL HAL_TimerIRQClear

        Pull    "r0" ; get device number back
        CallHAL HAL_IRQClear
        Pull    "r9,r12"
 |
        Push    r14

        MOV     R0, #timer0_bit
        STRB    R0, [R3, #IOCIRQCLRA]   ; clear timer 0 interrupt
 ]

        LDR     R1, =ZeroPage
        LDR     R0, [R1, #MetroGnome]
        ADD     R0, R0, #1
        STR     R0, [R1, #MetroGnome]

        LDRB    R0, CentiCounter        ; Counter for VDU CTRL timing
        SUBS    R0, R0, #1
        STRCSB  R0, CentiCounter        ; decrement if not zero

        LDR     R0, IntervalTimer +0
        ADDS    R0, R0, #1              ; Increment the low 4 bytes
        STR     R0, IntervalTimer +0

        LDREQB  R0, IntervalTimer +4
        ADDEQ   R0, R0, #1              ; and carry into 5th byte if necessary
        STREQB  R0, IntervalTimer +4

        Push    "R4,R12"                ; R0-R3 already pushed

        TEQEQ   R0, #&100               ; has interval timer crossed zero ?
        MOVEQ   R0, #Event_IntervalTimer ; Event ITCZ
        BLEQ    OSEVEN

        BL      CentiSecondTick         ; Notify keyboard of a centisecond

        Pull    "R4,R12"

        LDR     R0, RealTime +0         ; Increment 5-byte real time
        ADDS    R0, R0, #1
        STR     R0, RealTime +0
        LDRCSB  R0, RealTime +4
        ADDCS   R0, R0, #1              ; Won't wrap until 2248 and then it
        STRCSB  R0, RealTime +4         ; all falls over anyway

        LDRB    R0, TimerState          ; get switch state
        TEQ     R0, #5                  ; toggles between 5 and 10

        LDREQ   R1, TimerAlpha +0       ; either load from one
        LDREQB  R2, TimerAlpha +4

        LDRNE   R1, TimerBeta +0        ; or the other
        LDRNEB  R2, TimerBeta +4

        ADREQ   R3, TimerBeta +0        ; and point to t'other
        ADRNE   R3, TimerAlpha +0

        ADDS    R1, R1, #1              ; increment
        ADC     R2, R2, #0              ; with carry

        STR     R1, [R3]                ; and store back
        STRB    R2, [R3, #4]

        EOR     R0, R0, #&0F            ; 5 <-> 10
        STRB    R0, TimerState

        Push    R10

        MOV     R10, #TickerV           ; call 100Hz vector
        BL      CallVector              ; IRQ's still disabled

        BL      ProcessTickEventChain   ; Re-enables IRQs

        Pull    "R10,PC"

; ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Process VSync IRQ device
; ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

        ALIGN 32
FalseVsyncIRQ ROUT
        LDR     R1, =ZeroPage
        LDR     R0, [R1, #MetroGnome]
        TST     R0, #1
        MOVEQ   pc, lr
        Push    "lr"
        ; Fall through...

VsyncIRQ_ExtEntry ROUT
        LDRB    R0, CFStime             ; decrement 'CFS' timer !
        SUB     R0, R0, #1
        STRB    R0, CFStime

        VDWS    WsPtr                   ; Do our stuff before issuing VSYNC event
        BL      VsyncCall
        BYTEWS  WsPtr

        MOV     R0, #Event_VSync        ; VSYNC event number
        BL      OSEVEN

        LDRB    R1, FlashCount
        SUBS    R1, R1, #1
        Pull    PC, CC                  ; was zero, so frozen
        STRNEB  R1, FlashCount          ; else if now non-zero, store it back
        Pull    PC, NE                  ; not time to flash yet

        LDRB    R1, FlashState          ; Get the state and
        EORS    R1, R1, #1              ; flip to the other one (setting flags)
        STRB    R1, FlashState

        LDREQB  R2, SpacPeriod          ; get appropriate new period
        LDRNEB  R2, MarkPeriod
        STRB    R2, FlashCount          ; and put into counter

        VDWS    WsPtr
        Push    R4
        BEQ     dothesecondflash

dothefirstflash
        BL      DoFirstFlash
        Pull    "R4, PC"

dothesecondflash
        BL      DoSecondFlash
        Pull    "R4, PC"

; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

        END