I have developed a small Single Board computer, with a few customizations..
I am well versed in Assembly programming for 8052, but not for C. I have written driver functions for the devices on the board in assembly and this "BIOS" will be burned in the on-chip 20KB (89c55) eeprom as a simple monitor program. Upon startup, it will initialize some variables, a custom protocol stack and then jump to user program rom (at 8000h) address.
I want to write user codes in C, but in SMALL and / or COMPACT modes, since C uses the internal data ram for stack and other variables, this might clash with the BIOS variables that are at fixed internal-ram addresses.
Secondly, the BIOS program will initialize stack to reside in the extra 128-bytes ram (80h and above) while it is not certain where C will put the stack to start at.
Can anyone guide me as to how this situation can be avoided ?
If you really are well versed in Assembly programming for 8052, then you will find (nearly) all the answers to your questions by studying the startup code of the C application. It is written in assembler.
Hint#1 - You will find that it IS certain where C puts the stack.
Hint#2 - You will have to control where the linker places the relevant sections to avoid any nasty clashes.
Thanks Arthur.
One thing though...i am not using the AS51 assembler provided by Keil...i use Intel's assembler. SO the documentation specific to this is uite different. Do i have to lookup the documentation of teh assembler shipped with Keil ?
Or shud i compile a C code and then do reverse debug engineering to see the stack locations ?
"a custom protocol stack ... I want to write user codes in C"
When this stack is called from the C user code, the stack subroutine should follow the C calling convention. You have to read carefully the sections of C manual starting from this page, to know how to configure your asm code to fit to C convention.
Interfacing C to Assembler http://www.keil.com/support/man/docs/c51/c51_ap_ctoasm.htm
Also this kb will help you.
C51: CALLING ASSEMBLY ROUTINES FROM C http://www.keil.com/support/docs/50.htm "since C uses the internal data ram for stack and other variables, this might clash with the BIOS variables that are at fixed internal-ram addresses."
Apply DATA and IDATA linker directive to the C user code, so that the C working domain doesn't overlap your "BIOS" variables on the absolute address.
DATA Linker Directive http://www.keil.com/support/man/docs/bl51/bl51_data.htm IDATA Linker Directive http://www.keil.com/support/man/docs/bl51/bl51_idata.htm
Tsuneo
"One thing though...i am not using the AS51 assembler provided by Keil...i use Intel's assembler. SO the documentation specific to this is uite different. Do i have to lookup the documentation of teh assembler shipped with Keil ?"
The Keil assembler is similar to the Intel one - The opcodes (of course) are the standard 8052 ones and are therefore the same.
"Or shud i compile a C code and then do reverse debug engineering to see the stack locations ?"
You previously said you wanted to write user code in C.
Is that Keil C?
If so, then you have access to the documentation.
If not, then why are you asking these questions on a Keil forum?
The details relating to stack and data placement are going to be very specific to the compiler vendor; therefore I think you must decide on the vendor before making any more decisions.
You don't need to worry about the stack of the user's C code so much, when you ensure your BIOS variable area using DATA and IDATA linker directive on the user's C code.
- On the startup, your BIOS runs. You can place the stack top anywhere as you like.
- When your BIOS jumps to the user's C code, the C initialization in startup.A51 sets up the stack for itself. With DATA and IDATA directive, this stack location is aware of your BIOS variable.
- When the C code calls your BIOS routine, it uses the stack set by the C code.
WEll, thanks guys.
Arthur.. the development environment and cross-compiler are selected to be Keil C..so that not an issue. Maybe i wasnt able to explain in detail. I am relatively new to C programming for 8052 environment, and specifically to Keil, so yes i might have missed a few key points in Keil's documentation, and thanks for the tips / pointer.
Tsuneo..thanks for the descriptive message, i guess i can handle the rest from here. Incase something goes wrong or i cant understand.. i'll let u guys know so u can fill in. Thanks a lot.
One more thing though..the custom protocol stack wont be called by the User C code. The BIOS codes specific to the protocol handling will recieve a valid packet from the serial lines, and only if the packet is intended for a particular control card (address matching with address stored in serial EEPROM 24cxxx series) then a DATA RECIEVED flag wud be set which the user code can uery by wrapper functions and then read the packet data.
Likewise, if the user C code has to reply with a packet..it would send teh packet data to the BIOS functions by wrapper functions, which would actually do the transmitting.
I will make available the BIOS routines and protocol stack once it is finalized, for any one's review or usage online.
Thanks again.
"the custom protocol stack wont be called by the User C code... it would send teh packet data to the BIOS functions by wrapper functions, which would actually do the transmitting."
Do you mean that some interrupt summons up your BIOS from the user's C code, and just the variables are exposed to the user's code as the common resource?
Then provide a library, in which the common variables are declared on absolute address using _at_ keyword.
If you don't like to expose some variables, but ensure the address, declare it as static variables in the library file.
For example,
volatile unsigned char idata bios_flag _at_ 0x80; // exposed to users static unsigned char idata reserved_area[15] _at_ 0x81; // not exposed
Absolute Variable Location http://www.keil.com/support/man/docs/c51/c51_le_absvarloc.htm
In this way, you can ensure the BIOS variables without any linker directive, like IDATA.