l.s

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include "x16.h"
#include "mem.h"

#define WRMSR		BYTE $0x0F; BYTE $0x30	/* WRMSR, argument in AX/DX (lo/hi) */
#define RDTSC 		BYTE $0x0F; BYTE $0x31	/* RDTSC, result in AX/DX (lo/hi) */
#define RDMSR		BYTE $0x0F; BYTE $0x32	/* RDMSR, result in AX/DX (lo/hi) */

#ifdef PXE
#define PDB		0x90000		/* temporary page tables (24KB) */
#else
#define PDB		0x08000
#endif PXE

#define NoScreenBlank	1
/*#define ResetDiscs	1*/

TEXT origin(SB), $0
	/*
	 * This part of l.s is used only in the boot kernel.
	 * It assumes that we are in real address mode, i.e.,
	 * that we look like an 8086.
	 *
	 * Make sure the segments are reasonable.
	 * If we were started directly from the BIOS
	 * (i.e. no MS-DOS) then DS may not be
	 * right.
	 */
	MOVW	CS, AX
	MOVW	AX, DS

#ifdef NoScreenBlank
	/*
	 * Get the current video mode. If it isn't mode 3,
	 * set text mode 3.
	 * Well, no. Windows95 won't co-operate here so we have
	 * to explicitly set mode 3.
	 */
	XORL	AX, AX
	MOVB	$0x0F, AH
	INT	$0x10			/* get current video mode in AL */
	CMPB	AL, $03
	JEQ	sayhello
#endif /* NoScreenBlank */
	XORL	AX, AX
	MOVB	$0x03, AL
	INT	$0x10			/* set video mode in AL */

sayhello:
	LWI(hello(SB), rSI)
	CALL16(biosputs(SB))

#ifdef ResetDiscs
	XORL	AX, AX			/* reset disc system */
	XORL	DX, DX
	MOVB	$0x80, DL
	INT	$0x13
#endif /* ResetDiscs */

#ifdef DOTCOM
/*
 *	relocate everything to a half meg and jump there
 *	- looks weird because it is being assembled by a 32 bit
 *	  assembler for a 16 bit world
 *
 *	only b.com does this - not 9load
 */
	MOVL	$0,BX
	INCL	BX
	SHLL	$15,BX
	MOVL	BX,CX
	MOVW	BX,ES
	MOVL	$0,SI
	MOVL	SI,DI
	CLD
	REP
	MOVSL

	/*
	 * Jump to the copied image;
	 * fix up the DS for the new location.
	 */
	FARJUMP16(0x8000, _start8000(SB))

TEXT _start8000(SB), $0
	MFSR(rCS, rAX)			/* fix up DS, ES (0x8000) */
	MTSR(rAX, rDS)
	MTSR(rAX, rES)

	/*
	 * If we are already in protected mode, have to get back
	 * to real mode before trying any privileged operations
	 * (like going into protected mode...).
	 * Try to reset with a restart vector.
	 */
	MFCR(rCR0, rAX)			/* are we in protected mode? */
	ANDI(0x0001, rAX)
	JEQ	_real

	CLR(rBX)
	MTSR(rBX, rES)

	LWI(0x0467, rBX)		/* reset entry point */
	LWI(_start8000(SB), rAX)	/* offset within segment */
	BYTE	$0x26
	BYTE	$0x89
	BYTE	$0x07			/* MOVW	AX, ES:[BX] */
	LBI(0x69, rBL)
	MFSR(rCS, rAX)			/* segment */
	BYTE	$0x26
	BYTE	$0x89
	BYTE	$0x07			/* MOVW	AX, ES:[BX] */

	CLR(rDX)
	OUTPORTB(0x70, 0x8F)
	OUTPORTB(0x71, 0x0A)

	FARJUMP16(0xFFFF, 0x0000)		/* reset */
#endif /* DOTCOM */

_real:

/*
 *	do things that need to be done in real mode.
 *	the results get written to CONFADDR (0x1200)
 *	in a series of <4-byte-magic-number><block-of-data>
 *	the data length is dependent on the magic number.
 *
 *	this gets parsed by conf.c:/^readlsconf
 *
 *	N.B. CALL16 kills rDI, so we can't call anything.
 */
	LWI(0x0000, rAX)
	MTSR(rAX, rES)
	LWI(0x1200, rDI)

/*
 *	turn off interrupts
 */
	CLI

/*
 *	detect APM1.2 bios support
 */
	/* save DI */
	SW(rDI, rock(SB))

	/* disconnect anyone else */
	LWI(0x5304, rAX)
	LWI(0x0000, rBX)
	INT $0x15

	/* connect */
	CLC
	LWI(0x5303, rAX)
	LWI(0x0000, rBX)
	INT $0x15
	CLI	/* apm put interrupts back? */

	JC noapm

	OPSIZE; PUSHR(rSI)
	OPSIZE; PUSHR(rBX)
	PUSHR(rDI)
	PUSHR(rDX)
	PUSHR(rCX)
	PUSHR(rAX)

	/* put DI, ES back */
	LW(rock(SB), rDI)
	LWI(0x0000, rAX)
	MTSR(rAX, rES)

	/*
	 * write APM data.  first four bytes are APM\0.
	 */
	LWI(0x5041, rAX)
	STOSW

	LWI(0x004d, rAX)
	STOSW

	LWI(8, rCX)
apmmove:
	POPR(rAX)
	STOSW
	LOOP apmmove

noapm:

/*
 *	end of real mode hacks: write terminator, put ES back.
 */
	LWI(0x0000, rAX)
	STOSW
	STOSW

	MFSR(rCS, rAX)			/* fix up ES (0x8000) */
	MTSR(rAX, rES)

/*
 * 	goto protected mode
 */
/*	MOVL	tgdtptr(SB),GDTR /**/
	 BYTE	$0x0f
	 BYTE	$0x01
	 BYTE	$0x16
	 WORD	$tgdtptr(SB)

	LWI(1, rAX)
	/* MOV AX,MSW */
	BYTE $0x0F; BYTE $0x01; BYTE $0xF0

/*
 *	clear prefetch queue (weird code to avoid optimizations)
 */
	/* JMP .+2 */
	BYTE $0xEB
	BYTE $0x00

/*
 *	set all segs
 */
/*	MOVW	$SELECTOR(1, SELGDT, 0),AX	/**/
	 BYTE	$0xc7
	 BYTE	$0xc0
	 WORD	$SELECTOR(1, SELGDT, 0)
	MOVW	AX,DS
	MOVW	AX,SS
	MOVW	AX,ES
	MOVW	AX,FS
	MOVW	AX,GS

/*	JMPFAR	SELECTOR(2, SELGDT, 0):$mode32bit(SB) /**/
	 BYTE	$0x66
	 BYTE	$0xEA
	 LONG	$mode32bit-KZERO(SB)
	 WORD	$SELECTOR(2, SELGDT, 0)

TEXT	mode32bit(SB),$0
	/*
	 *  make a bottom level page table page that maps the first
	 *  16 meg of physical memory
	 */
	MOVL	$PDB, DI			/* clear 6 pages for the tables etc. */
	XORL	AX, AX
	MOVL	$(6*BY2PG), CX
	SHRL	$2, CX

	CLD
	REP;	STOSL

	MOVL	$PDB, AX		/* phys addr of temporary page table */
	MOVL	$(4*1024),CX		/* pte's per page */
	MOVL	$((((4*1024)-1)<<PGSHIFT)|PTEVALID|PTEKERNEL|PTEWRITE),BX
setpte:
	MOVL	BX,-4(AX)(CX*4)
	SUBL	$(1<<PGSHIFT),BX
	LOOP	setpte

	/*
	 *  make a top level page table page that maps the first
	 *  16 meg of memory to 0 thru 16meg and to KZERO thru KZERO+16meg
	 */
	MOVL	AX,BX
	ADDL	$(4*BY2PG),AX
	ADDL	$(PTEVALID|PTEKERNEL|PTEWRITE),BX
	MOVL	BX,0(AX)
	MOVL	BX,((((KZERO>>1)&0x7FFFFFFF)>>(2*PGSHIFT-1-4))+0)(AX)
	ADDL	$BY2PG,BX
	MOVL	BX,4(AX)
	MOVL	BX,((((KZERO>>1)&0x7FFFFFFF)>>(2*PGSHIFT-1-4))+4)(AX)
	ADDL	$BY2PG,BX
	MOVL	BX,8(AX)
	MOVL	BX,((((KZERO>>1)&0x7FFFFFFF)>>(2*PGSHIFT-1-4))+8)(AX)
	ADDL	$BY2PG,BX
	MOVL	BX,12(AX)
	MOVL	BX,((((KZERO>>1)&0x7FFFFFFF)>>(2*PGSHIFT-1-4))+12)(AX)

	/*
	 *  point processor to top level page & turn on paging
	 *
	 *  this produces the apparently harmless "VMX|F(125):468 Dis 0x0:0x0"
	 *  message in the VMware log.
	 */
	MOVL	AX,CR3
	MOVL	CR0,AX
	ORL	$0X80000000,AX
	MOVL	AX,CR0

	/*
	 *  use a jump to an absolute location to get the PC into
	 *  KZERO.
	 */
	LEAL	tokzero(SB),AX
	JMP*	AX

/*
 * When we load 9load from DOS, the bootstrap jumps
 * to the instruction right after `JUMP', which gets
 * us into kzero.
 *
 * The name prevents it from being optimized away.
 */
TEXT jumplabel(SB), $0
	BYTE $'J'; BYTE $'U'; BYTE $'M'; BYTE $'P'

	LEAL	tokzero(SB),AX
	JMP*	AX

TEXT	tokzero(SB),$0
	/*
	 * Clear BSS
	 */
	LEAL	edata(SB),SI
	MOVL	SI,DI
	ADDL	$4,DI
	MOVL	$0,AX
	MOVL	AX,(SI)
	LEAL	end(SB),CX
	SUBL	DI,CX
	SHRL	$2,CX
	CLD
	REP
	MOVSL

	/*
	 *  stack and mach
	 */
	MOVL	$mach0(SB),SP
	MOVL	SP,m(SB)
	MOVL	$0,0(SP)
	ADDL	$(MACHSIZE-4),SP	/* start stack above machine struct */

	CALL	main(SB)

loop:
	JMP	loop

GLOBL	mach0+0(SB), $MACHSIZE
GLOBL	m(SB), $4

/*
 *  gdt to get us to 32-bit/segmented/unpaged mode
 */
TEXT	tgdt(SB),$0

	/* null descriptor */
	LONG	$0
	LONG	$0

	/* data segment descriptor for 4 gigabytes (PL 0) */
	LONG	$(0xFFFF)
	LONG	$(SEGG|SEGB|(0xF<<16)|SEGP|SEGPL(0)|SEGDATA|SEGW)

	/* exec segment descriptor for 4 gigabytes (PL 0) */
	LONG	$(0xFFFF)
	LONG	$(SEGG|SEGD|(0xF<<16)|SEGP|SEGPL(0)|SEGEXEC|SEGR)

	/* exec segment descriptor for 4 gigabytes (PL 0) 16-bit */
	LONG	$(0xFFFF)
	LONG	$(SEGG|(0xF<<16)|SEGP|SEGPL(0)|SEGEXEC|SEGR)

/*
 *  pointer to initial gdt
 */
TEXT	tgdtptr(SB),$0
	WORD	$(4*8)
	LONG	$tgdt-KZERO(SB)

/*
 * Output a string to the display.
 * String argument is in rSI.
 */
TEXT biosputs(SB), $0
	PUSHA
	CLR(rBX)
_BIOSputs:
	LODSB
	ORB(rAL, rAL)
	JEQ _BIOSputsret

	LBI(0x0E, rAH)
	BIOSCALL(0x10)
	JMP _BIOSputs

_BIOSputsret:
	POPA
	RET

/*
 *  input a byte
 */
TEXT	inb(SB),$0

	MOVL	p+0(FP),DX
	XORL	AX,AX
	INB
	RET

/*
 * input a short from a port
 */
TEXT	ins(SB), $0

	MOVL	p+0(FP), DX
	XORL	AX, AX
	OPSIZE; INL
	RET

/*
 * input a long from a port
 */
TEXT	inl(SB), $0

	MOVL	p+0(FP), DX
	XORL	AX, AX
	INL
	RET

/*
 *  output a byte
 */
TEXT	outb(SB),$0

	MOVL	p+0(FP),DX
	MOVL	b+4(FP),AX
	OUTB
	RET

/*
 * output a short to a port
 */
TEXT	outs(SB), $0
	MOVL	p+0(FP), DX
	MOVL	s+4(FP), AX
	OPSIZE; OUTL
	RET

/*
 * output a long to a port
 */
TEXT	outl(SB), $0
	MOVL	p+0(FP), DX
	MOVL	s+4(FP), AX
	OUTL
	RET

/*
 *  input a string of bytes from a port
 */
TEXT	insb(SB),$0

	MOVL	p+0(FP),DX
	MOVL	a+4(FP),DI
	MOVL	c+8(FP),CX
	CLD; REP; INSB
	RET

/*
 *  input a string of shorts from a port
 */
TEXT	inss(SB),$0
	MOVL	p+0(FP),DX
	MOVL	a+4(FP),DI
	MOVL	c+8(FP),CX
	CLD
	REP; OPSIZE; INSL
	RET

/*
 *  output a string of bytes to a port
 */
TEXT	outsb(SB),$0

	MOVL	p+0(FP),DX
	MOVL	a+4(FP),SI
	MOVL	c+8(FP),CX
	CLD; REP; OUTSB
	RET

/*
 *  output a string of shorts to a port
 */
TEXT	outss(SB),$0
	MOVL	p+0(FP),DX
	MOVL	a+4(FP),SI
	MOVL	c+8(FP),CX
	CLD
	REP; OPSIZE; OUTSL
	RET

/*
 *  input a string of longs from a port
 */
TEXT	insl(SB),$0

	MOVL	p+0(FP),DX
	MOVL	a+4(FP),DI
	MOVL	c+8(FP),CX
	CLD; REP; INSL
	RET

/*
 *  output a string of longs to a port
 */
TEXT	outsl(SB),$0

	MOVL	p+0(FP),DX
	MOVL	a+4(FP),SI
	MOVL	c+8(FP),CX
	CLD; REP; OUTSL
	RET

/*
 *  routines to load/read various system registers
 */
GLOBL	idtptr(SB),$6
TEXT	putidt(SB),$0		/* interrupt descriptor table */
	MOVL	t+0(FP),AX
	MOVL	AX,idtptr+2(SB)
	MOVL	l+4(FP),AX
	MOVW	AX,idtptr(SB)
	MOVL	idtptr(SB),IDTR
	RET

TEXT	putcr3(SB),$0		/* top level page table pointer */
	MOVL	t+0(FP),AX
	MOVL	AX,CR3
	RET

TEXT	getcr0(SB),$0		/* coprocessor bits */
	MOVL	CR0,AX
	RET

TEXT	getcr2(SB),$0		/* fault address */
	MOVL	CR2,AX
	RET

TEXT	getcr3(SB),$0		/* page directory base */
	MOVL	CR3,AX