ipxe/src/arch/i386/firmware/pcbios/gateA20.c

#include "realmode.h"
#include "timer.h"
#include "latch.h"
#include "bios.h"

#define K_RDWR		0x60		/* keyboard data & cmds (read/write) */
#define K_STATUS	0x64		/* keyboard status */
#define K_CMD		0x64		/* keybd ctlr command (write-only) */

#define K_OBUF_FUL	0x01		/* output buffer full */
#define K_IBUF_FUL	0x02		/* input buffer full */

#define KC_CMD_WIN	0xd0		/* read  output port */
#define KC_CMD_WOUT	0xd1		/* write output port */
#define KB_SET_A20	0xdf		/* enable A20,
					   enable output buffer full interrupt
					   enable data line
					   disable clock line */
#define KB_UNSET_A20	0xdd		/* enable A20,
					   enable output buffer full interrupt
					   enable data line
					   disable clock line */

enum { Disable_A20 = 0x2400, Enable_A20 = 0x2401, Query_A20_Status = 0x2402,
	Query_A20_Support = 0x2403 };

#define CF ( 1 << 0 )

#ifndef IBM_L40
static void empty_8042 ( void ) {
	unsigned long time;

	time = currticks() + TICKS_PER_SEC;	/* max wait of 1 second */
	while ( ( inb ( K_CMD ) & K_IBUF_FUL ) &&
		currticks() < time ) {
		/* Do nothing.  In particular, do *not* read from
		 * K_RDWR, because that will drain the keyboard buffer
		 * and lose keypresses.
		 */
	}
}
#endif	/* IBM_L40 */

/*
 * Gate A20 for high memory
 *
 * Note that this function gets called as part of the return path from
 * librm's real_call, which is used to make the int15 call if librm is
 * being used.  To avoid an infinite recursion, we make gateA20_set
 * return immediately if it is already part of the call stack.
 */
void gateA20_set ( void ) {
	static char reentry_guard = 0;
	unsigned int discard_a;
	unsigned int flags;

	if ( reentry_guard )
		return;
	reentry_guard = 1;

	__asm__ __volatile__ ( REAL_CODE ( "sti\n\t"
					   "stc\n\t"
					   "int $0x15\n\t"
					   "pushfw\n\t"
					   "popw %w0\n\t"
					   "cli\n\t" )
			       : "=r" ( flags ), "=a" ( discard_a )
			       : "a" ( Enable_A20 ) );


	if ( flags & CF ) {
		/* INT 15 method failed, try alternatives */
#ifdef	IBM_L40
		outb(0x2, 0x92);
#else	/* IBM_L40 */
		empty_8042();
		outb(KC_CMD_WOUT, K_CMD);
		empty_8042();
		outb(KB_SET_A20, K_RDWR);
		empty_8042();
#endif	/* IBM_L40 */
	}

	reentry_guard = 0;
}

void gateA20_unset ( void ) {
	/* Not currently implemented */
}