dragon-nx-32: tighten MPI detection

diff --git a/Kernel/platform-dragon-nx32/dragon.s b/Kernel/platform-dragon-nx32/dragon.s
index 18f0f71..7f1d92b 100644 (file)
--- a/Kernel/platform-dragon-nx32/dragon.s
+++ b/Kernel/platform-dragon-nx32/dragon.s
@@ -133,13 +133,23 @@ _mpi_set_slot:
 ;
 _mpi_present:
        lda 0xff7f      ; Save bits
+       tfr a,b
+       lsrb
+       lsrb
+       lsrb
+       lsrb
+       eorb 0xff7f
+       andb #0x03      ; We expect to see the bits 5-4 and 1-0 matching
+       bne nompi       ; not guaranteed but a good rule of thumb for us
        ldb #0xff       ; Will get back 33 from an MPI cartridge
        stb 0xff7f      ; if the emulator is right on this
        ldb 0xff7f
+       andb #0x33
        cmpb #0x33
        bne nompi
        clr 0xff7f      ; Switch to slot 0
        ldb 0xff7f
+       andb #0x33      ; We can't trust the high bits
        bne nompi
        incb
        sta 0xff7f      ; Our becker port for debug will be on the default