source: buchla-68k/orig/IOLIB/VCTRL.C@ 4b63bf3

/*
   =============================================================================
        vctrl.c -- VSDD control functions
        Version 7 -- 1988-08-15 -- D.N. Lynx Crowe
        (c) Copyright 1987,1988 -- D.N. Lynx Crowe

        unsigned int
        vbank(b)
        unsigned int b;

                Set VSDD Data Segment bank to b.
                Return old bank select value.


        vfwait()

                Wait for a FRAMESTOP update to transpire.
   =============================================================================
*/

#include "stddefs.h"
#include "hwdefs.h"
#include "vsdd.h"

/* 

*/

/*
   =============================================================================
        vbank(b) -- change VSDD Data Segment bank to b.  Return old bank.
        Assumes a 128K byte window, sets S15..S11 to zero.
   =============================================================================
*/

unsigned int
vbank(b)
unsigned int b;
{
        register unsigned int   newb, oldb;

        oldb = v_regs[5] >> 6;                  /* get old bank value */
        oldb = (oldb & 2) | ((oldb >> 2) & 1);

        if (oldb EQ b)                          /* done if same as b */
                return(b);

        newb = b << 6;                          /* calculate new bank value */
        newb = (newb & 0x0080) | ((newb << 2) & 0x0100);

        v_regs[5] = newb;                       /* set new bank value */

        while (v_regs[11] GT 300)  ;            /* wait for FRAMESTOP */
        while (v_regs[11] LT 175)  ;
        while (v_regs[11] GE 175)  ;

        return(oldb);                           /* return old bank value */
}

/* 

*/

/*
   =============================================================================
        vfwait() -- Wait for a FRAMESTOP update to transpire.
   =============================================================================
*/

vfwait()
{
        while (v_regs[11] GT 300) ;             /* wait for FRAMESTOP */
        while (v_regs[11] LT 175) ;
        while (v_regs[11] GE 175) ;
}