ACB.TYM%office-2@sri-unix.UUCP (07/29/83)
Can anyone direct me to where I might find out 1. What is in the first 6 bytes of the BDOS (before the entry point). 2. What significance is attached to the 4 addresses after the BDOS entry point (after the initial jump). What services are performed by the subroutines pointed to by these addresses. If this discussion has gone on before, I am sorry, just point me at the archive files are I will try to catch up. If there is a document available with with the answers, just point me. I got started on these questions after watching a popular directory lister (XD or SD) go south for the winter with a spooler overlay in place, and found it using these secret addresses. Thanks Bomberger@office-3.
rconn@brl@sri-unix.UUCP (07/29/83)
From: Rick Conn <rconn@brl> Hi -- 1. The first six bytes of the BDOS (before the entry point) contain the serial number. In CP/M systems, the serial number is contained within the CCP and the BDOS. 2. The four byte-pairs after the entry point are addresses of subroutines to be executed when certain BDOS errors occur. Specifically, these four byte pairs have the following meanings: 1st Pair: Address of Permanent Disk Error routine; this routine is executed when bad block is encountered 2nd Pair: Address of Select Error routine; this routine is executed when a select to an invalid disk (such as E when the BIOS allows for up to D) is made 3rd Pair: Address of Disk R/O Error routine; this routine is executed when a write to a R/O disk is attempted 4th Pair: Address of File R/O Error routine; this routine is executed when a write to a R/O file is attempted As a rule, these routines usually print an error message and then do a warm boot. Utilities may use these to trap out errors they cause and perform other functions besides warm boot. To summarize, the picture looks like this: Base of BDOS: DS 6 ;Serial Number JMP BDOSE ;Enter the BDOS at the command processor DW PERSUB ;Permanent Disk Error DW SELSUB ;Select Disk Error DW ROSUB ;R/O Disk Error DW RONSUB ;R/O File Error BDOSE: ... ;BDOS command processing Rick
RGF%mit-mc@sri-unix.UUCP (07/31/83)
From: Ronald G. Fowler <RGF@mit-mc> Date: 30 July 1983 18:50 EDT From: Ronald G. Fowler <RGF> To: ACB.TYM at OFFICE-2 cc: RGF Re: BDOS secrets 1) Those six bytes at the beginning are the CP/M serial number; they have little significance to anything other than the MOVCPM program (which disables interrupts and halts if these numbers do not match MOVCPM's internal serial number). I'm also told that some rather rude implementations of CP/M come with utilities that take it upon themselves to do this serial number check also. 2) The four vectors are pointers to the following routines (in order): a. Permanent error subroutine (ie, BDOS ERROR ON X: BAD SECTOR) b. Select error c. Disk read/only error d. File read/only error The BDOS uses these addresses to find the corresponding error- handling subroutine; in theory, a transient program could overlay this table with pointers to its own routines, and thereby circum- vent the unfriendly BDOS error handlers, replacing the routines with more human-oriented messages. In practice, few programs do this; it's a risky practice, and not quite as clear-cut as it sounds. --Ron Fowler
reece%nadc@sri-unix.UUCP (07/31/83)
The first three bytes are the CP/M serial number; the next three is a jump around the four addresses you mentioned. The four addresses are: BADSEC - BAD READ/WRITE HANDLER BADSEL - NON-EXISTENT DRIVE HANDLER DISKRO - DISK READ-ONLY FILERO - FILE READ-ONLY
reece%nadc@sri-unix.UUCP (08/01/83)
I have been corrected--the first SIX bytes of the BDOS are the serial number, not three as I stated earlier.
CCVAX.ty%nosc@sri-unix.UUCP (08/01/83)
From: Ty Wernet <CCVAX.ty@nosc> The first 6 bytes in the BDOS is the serial number. The 4 address you referred to follow: Permanent error subroutine "Bad Sector" Select error subroutine "Select Error" Read-only Subroutine "R/O Bdos error on " Read-only error subroutine "File R/O Bdos error on "
goldfarb.ucf-cs%rand-relay@sri-unix.UUCP (08/07/83)
Have you ever noticed that when MOVCPM halts with a serial number
disparity, it prints:
SYNCRONIZATION [sic] ERROR
This goes back as far as I can remember (version 1.?). Maybe they
were looking for one byte to save in MOVCPM and dumped the 'H'? :-)
BenRGF@mit-mc@sri-unix.UUCP (08/07/83)
From: Ronald G. Fowler <RGF@mit-mc> Date: 1 August 1983 02:52 EDT From: Ronald G. Fowler <RGF> To: goldfarb.ucf-cs at RAND-RELAY cc: RGF Re: [RGF: BDOS secrets] Naw, I have the source, and there is no reason to need to save a byte; it's just a case of nott being abel to spel corecttly. --Ronn