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Heathkit H-100 functions to manage 2 video display pages, in DeSmet C

The Heathkit H-100 (Zenith Z-100) computer has 2 video pages. These DeSmet C functions allow you to:

Independently choose which page is written to and which one is currently displayed.
Draw to one page while the other is displayed, and then flip them.

These functions directly access the H-100 hardware and will not work on PC compatibles or any other computer that I know of.

/* ===========================================================*/
/* H100 VIDEO DISPLAY MANAGEMENT FUNCTIONS FOR MULTIPLE DISPLAY PAGES. */
/* THERE ARE 2 VIDEO PAGES. YOU CAN INDEPENDENTLY CHOOSE WHICH IS WRITTEN TO */
/* AND WHICH IS CURRENTLY DISPLAYED */
/* Copyright (C)1993 Steven Whitney. Published under the */
/* GNU GPL (General Public License) Version 3, with ABSOLUTELY NO WARRANTY. */
/* Initially published by http://25yearsofprogramming.com. */
/* ===========================================================*/
/* -> These functions are highly specific to the H100 computer hardware, */
/* -> and could have disastrous consequences on any other computer. */
/* -------------------------------------------------------------- */
/*	FLIPCPU() 10-12-93 */
/*	determines which page VRAM WRITES go to */
/*	toggles bit 7 of address latch, port DA */
/*  toggles between pages 1 & 2 as seen by CPU */
/* -------------------------------------------------------------- */
int flipcpu()
{
	char _inb();
	void _outb();
										/* get value in DA */
	_outb(((_inb(0xDA))^(0x80)),0xDA);	/* XOR with 80h & write back */
	return 1;
}

/* -------------------------------------------------------------- */
/*	FLIPCRTC() toggles between pages 1 & 2 as seen by CRTC */
/*	determines which page is DISPLAYED on the screen */
/*	toggles bit 3 of R12, the high byte of the start address register */
/*	R12 bit 3 = bit 11 of word R12-R13, and corresponds to */
/*	bit 7 of the address latch */
/*  10-12-93 */
/* -------------------------------------------------------------- */
int flipcrtc()
{
	char _inb();
	void _outb();

	_outb(12,0xDC);					/* write 12 to pointer register */
	_outb((_inb(0xDD)^8),0xDD);		/* XOR contents with 8 & write back */
	return 1;
}

/* -------------------------------------------------------------- */
/*	RESTOREPG1() sets both CPU and CRTC to video page 1 */
/*	emergency exit used in case I can't figure out which page is which */
/*  10-12-93 */
/* -------------------------------------------------------------- */
int restorepg1()
{
	char _inb();
	void _outb();
											/* get value in DA */
	_outb(((_inb(0xDA))&(~(0x80))),0xDA);	/* turn off bit 7 & write back */
	_outb(12,0xDC);							/* write 12 to pointer register */
	_outb((_inb(0xDD)&(~8)),0xDD);			/* turn off bit 3 & write back */
	return 1;
}