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Bootstrap Loader DJ/DMA

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  by Bill Degnan - 02/11/2009 22:20
Using Bob Grieb's Z80 ICE v 0.62 I located the bootstrap loader in memory. Here is how I did it. The controller was installed in the known-working California Computer Systems 2200 referenced on this web site.

1. Removed Z80 of the Jade CPU, and in its place inserted the Z80 ICE. Used terminal program to read output to screen.

2. Power on with ICE pre-set to EA ("cpu running") mode. Screen returns:

0000 00 D
0000 00 D
0000 00 D
INBRA Running

3. D 0000

message appears: "stop CPU to use that command"

4. Q (to stop CPU)

5. D 0000

returns:
00 3E 00 D3 08 C3 00 E8 etc. ("means jump to E800")

6. D E800

returns:
FF FF FF FF FF FF FF FF FF FF etc.

(after moving the code from E800 to another location)

7. D 0000

returns:
FF FF FF FF FF FF FF FF FF FF etc.

-----
BUT
-----

If one repeats steps 2-7 above omitting step 6, the Jade CPU does have a chance to clear the RAM from 0000, and therefore the bootstrap code for the drive controller remains intact, starting at 0038:

21 4A 00 LXI H,4A
36 MVI M,0
7E MOV A,M
B7 ORA A
CA 3D 00 JZ LOOP
FE 40 CPI 40H
C2 3D 00 JNZ LOOP
C3 80 00 JMP 80H
FF DB OFFH

Which I believe that the bootstrap checks an 8 in. drive, then for a 5 1/4"

The bottom line is that even without UART (RS232 terminal interface) I was able to confirm memory, z80 operation, and now the drive controller status.

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