For the Retrochallenge 2019/03, I am continuing the work I started in RC 2018/09 and trying to finish KittyOS into a real operating system, finally making the CAT-644 a real computer.
This project is the creation of an operating system for the Cat-644. (external link to hackday.io) The hardware is a 20 Mhz 8-bit homebrew computer built around an AtMega 644 microcontroller, and features VGA video output. Retrochallenge main page
A basic implemention of halloc, hgrab, and hrelease is working! I was able to allocate external memory, put data in it, then recall it later by handle. The system now has 2K of SRAM user heap 'cache', and 64k of XRAM.
/* This is a basic test of halloc, and handle-based SRAM swapping /*
char* s = "This is a string";
char * hp;
char * hp2;
//allocate memory, create a handle for it
u16 h = halloc(strlen(s)+1);
/* HP and HP2 should be the same pointer */
hp = hgrab(h);
hp2 = hgrab(h); //get again
DMESGF("%x %x\r\n", hp, hp2);
strcpy(hp, s); //copy it
/* release both grabs on the item */
hrelease(hp); /* should dev ref count in sram */
hrelease(hp2); /* should copy & destroy the buffer */
char* other = mmalloc(10); /* allocate memory. Should 'steal' the pointer that was released above */
DMESGF("%p\r\n",other);
hp = hgrab(h); /* Grab the string by handle */
DMESGF("%p %s\r\n",hp); /* Should print 'This is a test', and it should be at a different physical address than before. */
OUTPUT Meaning ------------------------------------------------------------------------------------------------------------- new handle 8001 created object 8001 not found in heap first grab: object not in internal heap yet xobject size 18 copy from x copy from external memory (is uninitialized garbage) 8001 found in heap second grab: object still in heap, just add to refcount hp 0xb52 hp2 0xb52 shows both grabbed pointers are same physical address found handle 8001 first release found handle 8001 second release last reference, copy to x the second release trigger writing to external memory other 0xb52 a different malloc took our old physical address (b52 above) 8001 not found in heap grab the object xobject size 18 copy from x swap into internal ram hp b5e This is a string string still intact; at a different physical address
There are a lot of optimizations that can be done in the future. The last release doesn't have to trigger a copy to external ram; not until another mmalloc needs memory. If an object is only read and not modified, there does not need to be a copyback. Perhaps a mmalloc can succeed, but if a released object is moved instead of copied back. There are a bunch of scenarios, but I am not going to implemenet any of them until actually needed. I just need to keep them in mind so I don't take the design somwhere where the optimizations become impossible.