System information

If you want to locate the address x within the memory image loaded under DDT in a 20K
systei-n, first type
Hx,n Hexadecimal sum and difference
and DDT responds with the value of x + n (sum) and x - n (difference). The first number printed
by DDT is the actual memory address in the image where the data or code is located. For
example, the following DDT command:
H3400,D580
produces 980H as the sum, which is where the CCP is located in the memory image under DDT.
Type the L command to disassemble portions of the BIOS located at (4A00H + b) - n, which,
when one uses the H command, produces an actual address of 1F80H. The disassembly
command would thus be as follows:
L1F80
It is now necessary to patch in the CBOOT and CBIOS routines. The BOOT resides at location
0900H in the memory image. If the actual load address is n, then to calculate the bias (in), type
the command:
H900,n Subtract load address from target address.
The second number typed by DDT in response to the command is the desired bias (in). For
example, if the BOOT executes at 0080H, the command
H900,80
produces
0980 0880 Sum and difference in hex.
Therefore, the bias in would be 0880H. To read-in the BOOT, give the command:
ICBOOT.HEX Input file CBOOT.HEX
Then
Rm Read CBOOT with a bias of in (= 900H - n).
6.3 Second-level Regeneration CP/M Operating System Manual
6-9