Btw you can just allocate an array of 31 FIBS. It is not possible to got more than 31 levels deeps, because max path is 63.
My assembly is broken. Somehow the program terminates after walk("*.*"). When I run this on a dir as floppy, I sometime get the question to change the system time.
while (!ret) {
if (fibStore->Filename[0] != '.') {
// debug -> address of the FIB structure
String_Format(outBuf,"%i\n\r$",fibStore);
DOS_StringOutput(outBuf);
// display filename
String_Format(outBuf,"%s\n\r$",fibStore->Filename);
DOS_StringOutput(outBuf);
// is a directory?
if ((fibStore->Attribute & 0x10) == 0x10 ) {
DOS_ChangeDir(fibStore->Filename);
walk("*.*");
DOS_StringOutput("Are we back?$"); //<- this line will never be executed
}
}
//fib = DOS_FindNextEntry();
ret = FNext(fibStore);
} 
I think there is a confusion in the use of Mem_HeapAlloc / Mem_HeapFree function.
As said before, it is a flat allocator; extremely fast, but that can only be used for static memory allocation or temporary buffer.
Mem_HeapFree() takes a size as a parameter while you pass it a pointer.
If you are sure that all the allocations are linear (which should be the case with a recursive directory tree), you can use this function with the size of the FIB structure.
Otherwise you can use the dynamic allocator which is more expensive (even if I optimized it for the z80) but works like the one in C.
There is an example of use in the sample s_sys.
The working code
//**************************************************************************
//* recursively walk a directory tree
//* dialect : MSXgl
//**************************************************************************
#include
#include
#include
// at this location there is BASIC buffer which is unused, use for 256Bytes
u8 __at(0xF55E) outBuf[256];
// Change Directory
u8 DOS_ChangeDir(const c8* path)
{
path; // HL
__asm
ex de,hl
ld c, #DOS_FUNC_CHDIR // DOS routine
call BDOS
ret
__endasm;
}
// Find First File
// requires a *FIB so each recursion can have its own FIB
// c8* search pattern
u8 FFirst(DOS_FIB* fib,const c8* param)
// fib HL
// param DE
{
fib; //HL
param; //DE
__asm
push ix
ld b,#0x37 // attributes
ld c,#0x40 // Find First File
push hl
pop ix
call BDOS
pop ix
__endasm;
}
// continue search with *FIB
u8 FNext(DOS_FIB* fib)
// fib HL
{
fib; //HL
__asm
push ix //save ix
push hl
pop ix
xor b // b == 0
ld c,#0x41 // fnext
call BDOS
pop ix
__endasm;
}
void walk(const u8* param)
{
u8 ret;
Mem_Set('$',outBuf,15);
DOS_FIB* fibStore =(DOS_FIB*)Mem_HeapAlloc(sizeof(DOS_FIB));
ret = FFirst(fibStore,param);
while (!ret) {
if (fibStore->Filename[0] != '.') {
String_Format(outBuf,"%s\n\r$",fibStore->Filename);
DOS_StringOutput(outBuf);
if ((fibStore->Attribute & 0x10) == 0x10 ) {
DOS_ChangeDir(fibStore->Filename);
walk("*.*");
DOS_ChangeDir("..");
}
}
ret = FNext(fibStore);
}
Mem_HeapFree(sizeof(DOS_FIB));
}
void main()
{
walk("*.*"); // this is the current directory
}