; From Dr. Dobb's Journal, Number 73, November 1982, pp. 6
; A letter from Oscar Goldman, extracted from
; CP/M EXCHANGE by Gene Head
; Dear Gene,
; This scheme is based on intercepting calls to the BDOS. The
; only visible entry point into the BDOS is the jump instruction
; at the absolute memory address of 5. WHile it might seem a
; simple matter to change the address at 6 and 7 in order to
; effect the interception, many applications programs (e.g., DDT)
; determine memory size from the information contained there.
; Therefore, it would seem prudent to leave the content of 6 and
; 7 alone. Fortunately, for our purpose, the instruction jumped
; to from 5 is also a jump instruction. This observation is the
; key to the interception process.
; Here is an outline of the modifications to be made in the
; BIOS. (If necessary, change the identifiers to avoid
; duplication.)
; Add the following two equates:
; MAXFUN EQU 49 ; Last of the standard BDOS function numbers
; EXTRA EQU ? ; Replace the "?" with the actual number
; of new BDOS calls. The new function
; numbers will start from 50.
; Toward the end of the warm boot, there is a routine called
; "gocpm". (This name is used by Digital Research in the listing
; of the skeletal CBIOS.) Somewhere in there will be found the
; two lines:
; LXI H,BDOS
; SHLD 6
; Immediately after these, insert the following 4 lines:
; and finish with the rest of "gocpm".
; At a convenient point in the BIOS, add the following:
NORMAL: EQU JMP 0 ; The 0 will be replaced during
; the warm boot.
DIVERT:
; This point is reached with the function number in C
; and the BDOS call information in DE.
MVI A,MAXFUN
CMP C ; Is it an old or new call?
JNC NORMAL ; Old ones are diverted to NORMAL.
ADI EXTRA
CMP C ; Are we exceeding the number of
; allowed calls?
JNC OKAY ; Continue if not.
LXI H,0 ; Protocol requires these registers
; to be zeroed for out-of-range calls.
RET
;
OKAY:
; Here we handle the new functions
LXI H,0 ; HL is lost.
DAD SP
SHLD SAVSTK ; Save the stack pointer for later return,
LXI SP,SAVSTK ; and set up a local stack.
XCHG
SHLD NTRPAR ; Save the entry parameters.
LXI D,FIN
PUSH D ; A place to return to when done.
MOV A,c
SBI MAXFUN+1 ; Start counting from 0.
ADD A ; Double it
MOV E,A
MVI D,0
LXI D,TABLE
DAD D ; HL points to the revelent table entry.
MOV E,M ; Get the address
INX H ; contained there,
MOV D,M ; and move it
XCHG ; into HL.
PCHL ; Go do it.
;
FIN:
; Return to here when done.
LHLD SAVSTK
SPHL ; Restore old stack pointer.
LHLD XITPAR ; Get the result of the routine.
MOV A,L
MOV B,H ; As per CP/M protocol.
RET ; All done.
;
TABLE:
; Jump table for the various new BDOS calls.
; There should be one entry for each of "EXTRA" items.
NEW0: DW DONEW0
NEW1: DW DONEW1
...
...
;
NITPAR: DS 2 ; Save the entry value of DE.
XITPAR: DS 2 ; Save the result of the BDOS call.
DS 40 ; Ample local stack -
; change this if necessary.
SAVSTK: DS 2 ; Save the stack pointer on entry.
;
; Each of the following routines must end with a RET instruction.
; If the routine returns a value, then it should be stored in XITPAR.
;
DONEW0: ; Actual routine #0.
