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Internals: Memory map
Fudge is very specific in how it handles memory. Because there is no memory allocation there has to be a precise map of what memory is used and for what purpose. In the current statte Fudge should run with just 16MB of memory but this might change in the future. With this layout Fudge can handle 64 running tasks.
| Base | Limit | Size | Name |
| 0×00000000 | 0×00100000 | 0×00100000 | BIOS Reserved |
| 0×00100000 | 0×00280000 | 0×00180000 | Kernel code + Ramdisk |
| 0×00280000 | 0×00300000 | 0×00080000 | Kernel stack |
| 0×00300000 | 0×00340000 | 0×00040000 | Page directories |
| 0×00340000 | 0×00380000 | 0×00040000 | Page tables for kernel code |
| 0×00380000 | 0×003C0000 | 0×00040000 | Page tables for task code |
| 0×003C0000 | 0×00400000 | 0×00040000 | Page tables for task stack |
| 0×00400000 | 0×00800000 | 0×00400000 | Reserved for future use |
| 0×00800000 | 0×00C00000 | 0×00400000 | Task code |
| 0×00C00000 | 0×01000000 | 0×00400000 | Task stack |
This memory is reserved by BIOS and should not be touched. There are some gaps you could use but I prefer just to leave it alone.
This is where the Multiboot bootloader will place Fudge and the ramdisk during boot. I have reserved about 1.5MB of memory to hold them both.
The reason why the kernel stack is pretty large is because this might need to change later when I add support for multiple CPUs which would require a seperate stack per CPU. For a 64 CPU system the stack would be 8192 bytes per CPU which should be enough.
Holds page directories for paging. A page is 4096 bytes so this area can hold a total of 64 directories.
The kernel can reserve up to 64 locations in memory of 4MB each. These are mostly used by drivers to reserve memory mapped areas. These pages are mapped as read only for all tasks.
Each task gets one page table for its code but only 10 pages meaning a task code+data sections can not be larger than 64K.
The same limitations as the previous. Only 64k of stack per task.
Multicode, SMP stuff in the future probably need this area.
The area where a programs code and data sections are copied before running. A program can not exceed 64k of memory.
The task stack can be at most 64k.