From: Stuart Cheshire <[email protected]>
Subject: WriteThrough INIT source code
Date: 9 Apr 1994 17:28:11 GMT

// Disk cache bypass INIT
// Copyright (C) April 1994 Stuart Cheshire
// Should be compiled as an INIT resource
// (use the "custom header" option in Think C)

#include <Traps.h>

// I know an address should be defined as a pointer type (eg 'void *'),
// not a 'long', but Apple's Get & Set TrapAddress calls are defined to
// use longs, so it's not my fault

typedef struct { unsigned short opcode; long addr; } JmpInstruction;

typedef void ShowIconFamily(short iconId);

void main(void);

static void entry_point(void)   /* no stack frame please ! */
       {
       asm     {
       @0              move.l  a4, -(sp)
                       move.l  a0, -(sp)
                       DetachResource
                       lea             @0, a0
                       move.l  a0, a4
                       bsr             main
       exit:   move.l (sp)+, a4
               }
       }

void my_write(void);  void sys_write(void);
static void my_write(void)
       {
       asm {
               tst.w   IOParam.ioRefNum(a0)                    ; Is this a file write?
               bmi.s   @sys_write
               cmp.l   #1024, IOParam.ioReqCount(a0)   ; Is it at leask 1K?
               blo.s   @sys_write
               ori.w   #0x20, IOParam.ioPosMode(a0)    ; Set "Don't cache" bit
       extern sys_write:
               jmp             0x12345678                                              ; Resume the system call
               }
       }

static Boolean TrapAvailable(unsigned long trap)
       {
       TrapType tType = (trap & 0x800 ? ToolTrap : OSTrap);
       if (trap & 0x800)                               // if it is a ToolBox Trap
               {
               unsigned long n = 0x400;        // number of toolbox traps
               if (NGetTrapAddress(_InitGraf, ToolTrap) == NGetTrapAddress(0xAA6E,
ToolTrap))
                       n = 0x200;
               if ((trap &= 0x7FF) >= n) trap = _Unimplemented;
               }
       return(NGetTrapAddress(trap, tType) != NGetTrapAddress(_Unimplemented,
ToolTrap));
       }

void main(void)
       {
       register long *p = &((JmpInstruction*)sys_write)->addr;
       *p = GetOSTrapAddress(_Write);
       SetOSTrapAddress((long)&my_write,_Write);
       if (TrapAvailable(_HWPriv)) FlushInstructionCache();
       }


Stuart Cheshire <[email protected]>
* <A HREF="file://brubeck.stanford.edu/www/cheshire-bio.html">WWW</A>
* Stanford Distributed Systems Group Research Assistant
* Escondido Village Resident Computer Coordinator
* Macintosh Programmer

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