Program Error w/ BDOS Function 9 (Print String)

[KKat]

Hey all,

I've been playing around with the YAZE implementation of CP/M, and have written a few simple programs. Now I'm trying to use BDOS function 9 to print a $-terminated string. The program assembles and runs, and I can see the terminated string in the hex dump, but when I make the call, it prints the string and then a bunch of garbage characters. The really odd thing is that it doesn't print the last character before the $ delimiter; for example:

"Hello, World!$" prints as "Hello, World" followed by garbage
"Hello, World$" prints as "Hello, Worl" followed by garbage

I think that for some reason, the CCP isn't seeing the $ delimiter (hence the garbage), but I can't figure out why, since it is correct in the hex dump. And I have NO idea why the last printable character doesn't print, regardless of string length.

Thanks in advance for any help.

KKat

P.S. I believe YAZE emulates v. 3.1; haven't written a program to check the version myself yet, though

 

[dhoelzer]

Is it possible that you've put the $ outside of the quotation marks? If you have, it is no longer terminating the string. It really sounds like the memory is being overwritten or that there is no $ to start with!

Hey all,

I've been playing around with the YAZE implementation of CP/M, and have written a few simple programs. Now I'm trying to use BDOS function 9 to print a $-terminated string. The program assembles and runs, and I can see the terminated string in the hex dump, but when I make the call, it prints the string and then a bunch of garbage characters. The really odd thing is that it doesn't print the last character before the $ delimiter; for example:

"Hello, World!$" prints as "Hello, World" followed by garbage
"Hello, World$" prints as "Hello, Worl" followed by garbage

I think that for some reason, the CCP isn't seeing the $ delimiter (hence the garbage), but I can't figure out why, since it is correct in the hex dump. And I have NO idea why the last printable character doesn't print, regardless of string length.

Thanks in advance for any help.

KKat

P.S. I believe YAZE emulates v. 3.1; haven't written a program to check the version myself yet, though

 

[ahm]

It would be useful for us to see your exact program.

 

[KKat]

Thanks for the replies; I think you are right about the $-terminator not being processed correctly, but I'm not sure why. Here is the code:


; Directives and defines

#charset ascii
#target rom

BOOT equ 0x0000 ; Default CP/M addresses
TPA equ 0x0100
BDOS equ 0x0005
print_string equ 0x09 ; BDOS function refs

#code _CODE, TPA

ld sp, stack ; Set up stack

ld c, print_string ; BDOS function 0x09
ld de, msg1 ; Point DE to string to print
call BDOS ; Print it.

jp BOOT ; Jump to BOOT to end program

; Define user input message

msg1:
.ascii 'Enter some text$'

; Allocate unused space at end to stack

stack:
end

 

[KKat]

I figured it out: by allocating the space right after the message string to the stack, I was overwriting the last two characters of the string when I called BDOS. I added two NOP instructions after the string (but before the "stack" label), and now the program runs correctly.

I know I don't need to allocate an internal stack for a simple program like this, but I wanted to get in the habit in preparation for longer programs. Where do you typically put the stack on a CP/M program? Is there a "standard" place to put it, or is it just up to the individual programmer?

Thanks,

KKat

 

[daver2]

The 'stack' is actually missing.

The stack pointer (SP) is set up to point to the label stack: and a call made to BDOS - which pushes the return address onto the stack (which doesn't exist) and promptly corrupts the last two bytes of the text message (t$) - hence the reason the BDOS function 'crashes' through memory.

There needs to be some uninitialised space between the end of your message and the label 'stack:' to actually contain the stack. The stack works its way down from it's initial point (not upwards).

Dave

 

[ahm]

Try changing your "jp BOOT" to "ret"

This works fine in CP/M:

        ORG     100H
        MVI     C,9
        LXI     D,MSG
        CALL    5
        RET
MSG:    DB      'HELLO WORLD$'

 

[KKat]

Thanks, everyone, for the replies; I got it to work.

KKat