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<html><head>	<title>Operating Systems FAQ :: C++ Programming</title>	<link rel=stylesheet type="text/css" href="default.css"></head><body><P>&nbsp;</P><TABLE border="0" width="100%">	<TR>		<TD><H2><A name="k6_writeback">AMD K6 WriteBack Optimisations</A></H2></TD>	</TR>	<TR>		<td>This source code uses GCC inline assembler (AT&amp;T syntax).		<p>I wrote and tested this on my own K6 (k6-200) and it works ok,		but I was unable to find anyone with a K6-2 (CXT core) or K6-3 since		there is two different methods for enabling writeback mode. It _should_		work fine on k6-2 CXT and K6-3 processors.		<p>With some tweaking, can be put into anyone's OS.		<p>You call AMD_K6_writeback with the CPUID results		family, model and stepping, only when you are sure		you have an AMD cpu.<pre>void AMD_K6_writeback(int family, int model, int stepping){    /* mem_end == top of memory in bytes */    int mem=(mem_end>>20)/4; /* turn into 4mb aligned pages */    int c;    union REGS regs;    if(family==5)    {        c=model;        /* model 8 stepping 0-7 use old style, 8-F use new style */        if(model==8)        {            if(stepping<8)                c=7;            else                c=9;        }        switch(c)        {        /* old style write back */        case 6:        case 7:            AMD_K6_read_msr(0xC0000082, &amp;regs);            if(((regs.x.eax>>1)&0x7F)==0)                kprintf(&quot;AMD K6 : WriteBack currently disabled\n&quot;);            else                kprintf(&quot;AMD K6 : WriteBack currently enabled (%luMB)\n&quot;,                    ((regs.x.eax>>1)&0x7F)*4);            kprintf(&quot;AMD K6 : Enabling WriteBack to %luMB\n&quot;, mem*4);            AMD_K6_write_msr(0xC0000082, ((mem<<1)&0x7F), 0, &amp;regs);            break;        /* new style write back */        case 9:            AMD_K6_read_msr(0xC0000082, &amp;regs);            if(((regs.x.eax>>22)&0x3FF)==0)                kprintf(&quot;AMD K6 : WriteBack Disabled\n&quot;);            else                kprintf(&quot;AMD K6 : WriteBack Enabled (%luMB)\n&quot;,                    ((regs.x.eax>>22)&0x3FF)*4);            kprintf(&quot;AMD K6 : Enabled WriteBack (%luMB)\n&quot;, mem*4);            AMD_K6_write_msr(0xC0000082, ((mem<<22)&0x3FF), 0, &amp;regs);            break;        default:    /* dont set it on Unknowns + k5's */            break;        }    }}void AMD_K6_write_msr(ULONG msr, ULONG v1, ULONG v2, union REGS *regs){    asm __volatile__ (        &quot;pushfl\n&quot;        &quot;cli\n&quot;        &quot;wbinvd\n&quot;        &quot;wrmsr\n&quot;        &quot;popfl\n&quot;        : &quot;=a&quot; (regs->x.eax),          &quot;=b&quot; (regs->x.ebx),          &quot;=c&quot; (regs->x.ecx),          &quot;=d&quot; (regs->x.edx)        : &quot;a&quot; (v1),          &quot;d&quot; (v2),          &quot;c&quot; (msr)        : &quot;eax&quot;,          &quot;ecx&quot;,          &quot;edx&quot;,          &quot;ebx&quot;,          &quot;memory&quot;);}void AMD_K6_read_msr(ULONG msr, union REGS *regs){    asm __volatile__ (        &quot;pushfl\n&quot;        &quot;cli\n&quot;        &quot;wbinvd\n&quot;        &quot;xorl %%eax, %%eax\n&quot;        &quot;xorl %%edx, %%edx\n&quot;        &quot;rdmsr\n&quot;        &quot;popfl\n&quot;        : &quot;=a&quot; (regs->x.eax),          &quot;=b&quot; (regs->x.ebx),          &quot;=c&quot; (regs->x.ecx),          &quot;=d&quot; (regs->x.edx)        : &quot;c&quot; (msr)        : &quot;eax&quot;,          &quot;ecx&quot;,          &quot;edx&quot;,          &quot;ebx&quot;,          &quot;memory&quot;);}</pre>		</TD>	</TR></TABLE></body></html>
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