typedef REG unsigned int;
REG* dma1Ctrl = ..., dma2Ctrl = ...;
REG* dma1Status = ..., dma2Status = ...;
REG* dma1Address = ..., dma2Address = ...;
REG* dma1Count = ..., dma2Count = ...;

struct IORequest{
    REG* buffer;
    unsigned size;
    int status;
    IORequest* next;
}
IORequest ioHead;

static IORequest* ioPending1 = null, ioPending2 = null;

void transfer() {
    if (!ioHead && ioPending1!=null) {
        ioPending1 = ioHead;
        startDMA1();
        ioHead = ioHead->next;
    }
    if (!ioHead && ioPending2!=null){
        ioPending2 = ioHead;
        startDMA2();
        ioHead = ioHead->next;
    }
}

void startDMA1() {
    *dma1Address = ioPending->buffer;
    *dma1Count = ioPending->size;
    *dma1Ctrl = 1;
}

void startDMA2() {
    *dma2Address = ioPending->buffer;
    *dma2Count = ioPending->size;
    *dma2Ctrl = 1;
}

interrupt void dmaInterrupt() {
    if (*dma1Status & 1) {
        ioPending1->status = *dma1Status;
        ioPending1 = null;
        transfer();
    }
    if (*dma2Status & 1) {
        ioPending2->status = *dma2Status;
        ioPending2 = null;
        transfer();
    }
}