Testing binary serial input in µVision4 debugger

I wrote a piece of code which uses small memory model. It compiles to 65 bytes data and 1646 bytes code. I'm using 3 register banks and all 16 bytes of bit addressable memory. My SP is initialized to address 30h. My main only initializes the hardware and goes into busy wait. I use timer interrupts to send output to parallel ports. The system receives 8-bit commands via serial port, and all commands are binary, like machine code.

My problem is that I can't send my commands to serial port in the debugger in µVision4. The serial window allows me to type to the serial port, but most of my commands do not map to typable characters in ASCII table. The serial tool allows me to trigger and interrupt by setting either RI or TI, but I can't set the contents of the buffer for receive interrupt.

The solution I came up with is to put a breakpoint in the serial interrupt routine, just after the contents of the buffer are read, and use watch window to set the variable into which it is read to the value of my desired command. However, my code is behaving erratically when I do this and I'm not sure whether this is because of the way I'm testing or some other error I've made. It works fine when I don't trigger any serial interrupt. Also, triggering a serial RI interrupt without touching any variables in watch window works without problems: the system receives and executes the 0x00 command. However, when I change a variable in watch window, it sometimes works as intended and sometimes jumps to a forbidden code memory location filled with zeroes.

Any pointers, either towards more reliable code or more reliable testing, would be much appreciated. Thanks in advance.

 assembler or C

My SP is initialized to address 30h.
risky business, where are your variables?

Erik

 RE: assembler or C

Yeah, quite. As it turns out, the compiler puts global variables and the locals it can't put into registers on the lowest memory addresses it thinks are available, and it doesn't take the location of stack into consideration. The reason for erratic behaviour of my code was the global were writing into stack. After some more additions to the code, ended up initializing SP to 60h.

So, my stability problem has been resolved, but I would still know if it is possible to type in a hex or binary value that would be send to the serial port as a byte.

 RE: assembler or C

the linker set the stack automatically after the last variable, why do you fool with it

 RE: assembler or C

I wasn't aware that the linker takes care of SP. Thanks. I had a leaky array in the first draft of my code so I assumed the problem was in SP initialization.

 RE: assembler or C

what does sp do?

 RE: assembler or C

Contains the memory address of the last occupied position on the stack.

 RE: assembler or C

i understand now. same like the register.

 RE: assembler or C

Yeah, you import reg51.h and you get a bunch of variables mapped to registers.

 RE: assembler or C

thx mate. you are ace.