Memory Organization

The following memory organization is used throughout the code. To the extent possible, these definitions have been collected in memorylayout.h, but changing any of them may require some some significant code surgery.

Physical Memory Layout

0000 0000 Reserved area for initial interrupt vector and BIOS data.
0000 7C00 (BOOTBLOCK_START) 512 bytes where bootblock.S is initially loaded.
0000 fff0 (INTERUPT_STACK_TOP) Initial location of kernel/interrupt stack
          until paging and user processes allocate their own stacks.
0001 0000 (KERNEL_START) Start of kernel code and data in kernelcore.S
0010 0000 (KMALLOC_START) Start of kernel memory managed by kmalloc().
0020 0000 (MAIN_MEMORY_START)  Start of memory pages managed by memory.c
???? ???? Location of the video buffer, determined by video BIOS at runtime.
          Care must be taken in memory allocation and pagetable setup
          to avoid stomping on this area.

Virtual Memory Layout

0000 0000 First 2GB of VM space for all processes is directly mapped to
          physical memory in kernel mode.  That way, kernel space is
          inaccessible in user mode, but kernel code can run correctly
          with paging activated.
8000 0000 (PROCESS_ENTRY_POINT) The upper 2GB of VM space for all processes
          is private to that process.  Each page of VM here is mapped to
          physical page allocated by memory.c.
ffff fff0 (PROCESS_STACK_INIT) The high end of the user space is designated
          for the user level stack, which grows down towards the middle
          of memory.

Segmentation

		Start	Length		Type	Privilege
Segment 1	0	4GB		CODE	SUPERVISOR
Segment 2	0	4GB		DATA	SUPERVISOR
Segment 3	0	4GB		CODE	USER
Segment 4	0	4GB		DATA	USER
Segment 5	tss	sizeof(tss)	TSS	SUPERVISOR

For technical reasons, the X86 requires that segmentation be turned on whenever paging is used. A few key places in interrupt handling and system startup require the use of segments. So, we set up four segments that span all of memory, and use the one with the desired privilege wherever a segment is required. The fifth segement identifies the Task State Structure (TSS) which is where the initial setup information for entering protected mode is stored.