286相關的東西

IBM 5170 手冊中 5-34頁：

IBM PC XT (286 BIOS)片斷

;-------------------------------------------------------------------------------
;  SHUTDOWN - RESTART							       :
;	RETURN CONTROL AFTER A SHUTDOWN COMMAND IS ISSUED		       :
;  DESCRIPTION								       :
;	A TEST IS MADE FOR THE SYSTEM FLAG BEING SET.  IF THE SYSTEM FLAG IS   :
;	SET, THE SHUTDOWN BYTE IN CMOS IS USED TO DETERMINE WHERE CONTROL IS   :
;	RETURNED.							       :
;									       :
;	CMOS = 0   SOFT RESET OR UNEXPECTED SHUTDOWN			       :
;	CMOS = 1   SHUT DOWN AFTER MEMORY SIZE				       :
;	CMOS = 2   SHUT DOWN AFTER MEMORY TEST				       :
;	CMOS = 3   SHUT DOWN WITH MEMORY ERROR				       :
;	CMOS = 4   SHUT DOWN WITH BOOT LOADER REQUEST			       :
;	CMOS = 5   JMP DWORD REQUEST - (INTERRUPT CHIPS & 287 ARE INITIALIZED) :
;	CMOS = 6   PROTECTED MODE TEST3 PASSED				       :
;	CMOS = 7   PROTECTED MODE TEST3 FAILED				       :
;	CMOS = 8   PROTECTED MODE TEST1 FAILED				       :
;	CMOS = 9   BLOCK MOVE SHUTDOWN REQUEST				       :
;	CMOS = A   JMP DWORD REQUEST - (W/O INTERRUPT CHIPS INITIALIZED)       :
;									       :
;	   NOTES:  RETURNS ARE MADE WITH INTERRUPTS AND NMI DISABLED.	       :
;		USER MUST RESTORE SS:SP (POST DEFAULT SET = 0000:0400),        :
;		ENABLE NON-MASKABLE INTERRUPTS (NMI) WITH AN OUT TO	       :
;		PORT 70H WITH HIGH ORDER BIT OFF, AND THEN ISSUE A	       :
;		STI TO ENABLE INTERRUPTS. FOR SHUTDOWN (5) THE USER	       :
;		MUST ALSO RESTORE THE INTERRUPT MASK REGISTERS. 	       :
;-------------------------------------------------------------------------------

;-----	CHECK FROM WHERE
C7C:
	MOV	AL,CMOS_SHUT_DOWN+NMI	; CLEAR CMOS BYTE
	OUT	CMOS_PORT,AL
	NOP				; I/O DELAY
	SUB	AL,AL			; SET BYTE TO 0
	OUT	CMOS_DATA,AL
	XCHG	AH,AL
	CMP	AL,0AH			; COMPARE WITH MAXIMUM TABLE ENTRIES
	JA	SHUT0			; SKIP TO POST IF GREATER THAN MAXIMUM
	MOV	SI,OFFSET BRANCH	; POINT TO THE START OF THE BRANCH TABLE
	ADD	SI,AX
	ADD	SI,AX			; POINT TO BRANCH ADDRESS
	MOV	BX,CS:[SI]		; MOVE BRANCH TO ADDRESS TO BX REGISTER

;----- SET TEMPORARY STACK FOR POST

	MOV	AX,ABS0 		; SET STACK SEGMENT TO ABS0 SEGMENT
	MOV	SS,AX
	MOV	SP,OFFSET @TOS		; SET STACK POINTER TO END OF VECTORS
	JMP	BX			; JUMP BACK TO RETURN ROUTINE

BRANCH: DW	SHUT0			; NORMAL POWER UP/UNEXPECTED SHUTDOWN
	DW	SHUT1			; SHUT DOWN AFTER MEMORY SIZE
	DW	SHUT2			; SHUT DOWN AFTER MEMORY TEST
	DW	SHUT3			; SHUT DOWN WITH MEMORY ERROR
	DW	SHUT4			; SHUT DOWN WITH BOOT LOADER REQUEST
	DW	SHUT5			; JMP DWORD REQUEST WITH INTERRUPT INIT
	DW	SHUT6			; PROTECTED MODE TEST7 PASSED
	DW	SHUT7			; PROTECTED MODE TEST7 FAILED
	DW	SHUT8			; PROTECTED MODE TEST1 FAILED
	DW	SHUT9			; BLOCK MOVE SHUTDOWN REQUEST
	DW	SHUTA			; JMP DWORD REQUEST (W/O INTERRUPT INIT)
PAGE
;-----	@IO_ROM_INIT MUST BE INITIALIZED BY THE USER FOR VECTORED REQUESTS

SHUT5:
	IN	AL,PORT_A		; FLUSH THE KEYBOARD BUFFER
	MOV	AL,EOI			; FLUSH LAST TIMER REQUEST IF PENDING
	OUT	INTA00,AL		;  - TO ALLOW TIMER INTERRUPTS
SHUTA:
	JMP	DWORD PTR @IO_ROM_INIT	; FAR JUMP TO USER DEFINED LOCATION

用05和0ah返回實模式在PCEM中能夠，用05返回須要恢復8259 的IMR，不然DOS不響應硬件中斷。0xa是INTEL推薦的方法，在MP手冊中推薦用於啓動和中止AP CPU（只限於使用82489DX的系統），這個技術已經被廢除了現代PC的BIOS基本上沒這個了。