User`s guide

ftest: ; is this function 12?
001B 79 mov a,c
001C FE0C cpi 12
001E CA2400 Jz begin ; Yes - intercept
0021 C30500 Jmp next ; some other function
begin:
0024 210000 lxi h,0
0027 39 dad sp ; save stack
0028 225400 shld ret$stack
002B 317600 lxi sp,loc$stack
002E 0E05 mvi c,pstring
0030 113E00 lxi d,test$msg ; Print message
0033 CD0900 call next ; call BDDS
0036 2A5400 lhld ret$stack ; restore user stack
0039 F9 sphl
003A 213100 lxi h,0031h ; return version number
003D C9 ret
test$msg:
003E 0D0A2A2A2A db cr,lf,'**** ECHDVERS ****$'
ret$stack:
0054 0000 dw 0
0056 ds 32 ; 16 level stack
loc$stack :
0076 end
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Construction of an RSX Program CP/M 3 Programmer's Guide