HIGHCODE.DOC

This demonstration employs several files:

	highdraw.s

	This file defines a 64-bit procedure that will be mapped 
	to a 'high' location in the CPU's virtual address space,
	and will simply draw a message directly to video memory,
	confirming that control did arrive at that location.  We
	selected the canonical address to be 0xFFFFFFFFC0000000,
	because (1) it will be straighforward to construct page-
	mapping tables for the page-frame at this location, and 
	(2) this address is the sign-extension of a 32-bit value
	so that our 'ld' linker can perform the code-relocation
	of 'symbolic addresses' (i.e., statement-labels) used.


	hiscript

	This is the linker-script needed for 'code-relocating',
	and it is used like this:

		$ ld highdraw.o -T hiscript -o highdraw.b



	highcode.s

	This program will load an image of the 'highdraw.b' file
	from a designated logical block of our '/dev/sda4' disk-
	partition into an area of physical memory that will then
	be 'mapped' to the page-frame at the high address in our
	program's virtual memory space; this program then enters
	16-bit 'compatibility mode', performs a direct long call
	(via a 64-bit call-gate) to the 64-bit procedure's entry
	point in the 'highdraw.s' program, and when that routine
	returns control to the 16-bit 'compatibility mode' code, 
	it will exit back to the 'real mode' code residing in an
	identity-mapped page-frame.

	
	ldscript

	This is the linker-script we have been using up till now
	to produce our previous the 'binary-format' executables,
	and we use it again here:

		$ ld highcode.o -T ldscript -o highcode.b


	NOTE: We have made a small modification to our previous
	boot-loader program ('cs686ipl.s') so that it passes to
	the 'highcode.s' application the starting Logical Block
	Address of our course's disk-partition ('/dev/sda4').