Volume Number: 13 (1997)
Issue Number: 5
Column Tag: Tips & Tidbits

Tips & Tidbits

By Steve Sisak

If you've got a program, such as a compiler, which creates a temporary file, writes and reads from it, and then no longer needs the data, don't just close it and delete it; doing so usually causes the temporary data to be written to disk. Before closing it, call SetEOF(fRefNum, 0); This tells the disk cache that none of the data associated with the file need ever be written.

Jorg Brown
brown@connectix.com

Here is a tip I bumped into recently. I wanted to run a sequence of m68k instructions (actually, to perform a system trap) on a PowerMac. Usually, this is a trivial task: compile the code in the 68k universe (for example, with in-line assembly supported by many m68k C/C++ compilers). From within a PowerPC application, one can call PowerPC glue code to the trap, which is probably present in the ‘InterfaceLib'.

Unfortunately, neither of these easy options would apply in my case: I only have a PowerPC compiler (I don't have disk space to install both 68k and PowerPC parts of the CodeWarrior). The trap I wanted to call (_ReadXPRam), is not an "official" one, thus there is no glue code for it in PowerPC libraries. Nevertheless, I found an easy way to run a sequence of 68k codes "compiled in" a PowerPC executable, and pass data in and out. By the way, this is working code that prints the contents of the xPRAM (more on xPRAM can be found at http://pobox.com/~oleg/ftp/xPRAM.html).

// Printing out the contents of the xPRAM
// The trick is that _ReadXPRam/_WriteXPRam traps are available only
// from within 68K universe. So, if this code runs in the PowerPC mode,
// we've got to switch universes before running M68K code sequences...

#include <stdio.h>
#include <MixedMode.h>

    // This is
    //    CLR.L   D0             address would be 0
    //    MOVE.W    $4(A7),D0    size -> lo word of d0
    //    SWAP  D0      size -> hi word of d0
    //    MOVEA.L  6(A7),A0where -> A0
    //    _ReadXPRam
    //    MOVEA.L (A7)+,A0 standard PASCAL epilogue
    //    ADDQ.W  #$6,A7
    //    JMP    (A0)
    //    RTS
    // This is a sequence of M68K instructions; unfortunately,
    // a PowerMac compiler doesn't understand them. So we've
    // got to assemble by hand <sigh>
    //
    // BTW, to write into xPRAM, replace 0xA051 in the sequence
    // below with 0xA052
    //pascal void read_extended_PRAM(char * where, const short size) =

static short read_extended_PRAM []=
{ 0x4280, 0x302F, 0x0004, 0x206F, 0x006, 0x4840, 0xA051, 0x205F, 0x5C4F, 
0x4ED0  };

#define COMP_NORET_2(name, a1, a2) \
 name##_procinfo = kPascalStackBased \
 | STACK_ROUTINE_PARAMETER(1, SIZE_CODE(sizeof(a1)))     \
 | STACK_ROUTINE_PARAMETER(2, SIZE_CODE(sizeof(a2)))

#define RD_ALLOC(routine) static RoutineDescriptor       \
 routine##_RD = BUILD_ROUTINE_DESCRIPTOR                 \
 (routine##_procinfo, routine)

enum {
  COMP_NORET_2(read_extended_PRAM,char *,const short)
};

void main(void)
{
  unsigned char whole_xPRAM_buffer[256];
  UniversalProcPtr u_read_extended_PRAM = 
   NewRoutineDescriptor((long (*)())read_extended_PRAM,
        read_extended_PRAM_procinfo,
        kM68kISA);
  CallUniversalProc(u_read_extended_PRAM,
  read_extended_PRAM_procinfo,
    whole_xPRAM_buffer,
  sizeof(whole_xPRAM_buffer));
  DisposeRoutineDescriptor(u_read_extended_PRAM);
 
  printf("\ncontents of the xPRAM\n");
  for(register int i=0; i<sizeof(whole_xPRAM_buffer); i+=16)
  {
    printf("\n%04x  ",i);
    for(register int j=0; j<16; j++)
   printf("%s%02x", j%4 == 0 ? " " : "", 
 whole_xPRAM_buffer[i+j]);
  }
}

Oleg Kiselyov
oleg@pobox.com