q800: add ESCC alias at 0xc000

Tests on real Q800 hardware show that the ESCC is addressable at multiple locations
within the ESCC memory region - at least 0xc000, 0xc020 (as expected by the MacOS
toolbox ROM) and 0xc040.

All released NetBSD kernels before 10 use the 0xc000 address which causes a fatal
error when running the MacOS booter. Add a single memory region alias at 0xc000
to enable NetBSD kernels to start booting under QEMU.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-19-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/m68k/q800.c

@@ -451,6 +451,12 @@ static void q800_machine_init(MachineState *machine)
     memory_region_add_subregion(&m->macio, SCC_BASE - IO_BASE,
                                 sysbus_mmio_get_region(sysbus, 0));
 
+    /* Create alias for NetBSD */
+    memory_region_init_alias(&m->escc_alias, OBJECT(machine), "escc-alias",
+                             sysbus_mmio_get_region(sysbus, 0), 0, 0x8);
+    memory_region_add_subregion(&m->macio, SCC_BASE - IO_BASE - 0x20,
+                                &m->escc_alias);
+
     /* SCSI */
 
     object_initialize_child(OBJECT(machine), "esp", &m->esp,

include/hw/m68k/q800.h

@@ -67,6 +67,7 @@ struct Q800MachineState {
     MemoryRegion macio;
     MemoryRegion macio_alias;
     MemoryRegion machine_id;
+    MemoryRegion escc_alias;
 };
 
 #define TYPE_Q800_MACHINE MACHINE_TYPE_NAME("q800")