From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Mon,  1 Feb 2016 17:47:46 +0100
Message-ID: <CAHL7psFg6JgatGw3hQb93kpkrZFGq7vtcz7eMkWWbM0cNfMGzA@mail.gmail.com>
From: Giacomo Tesio <giacomo@tesio.it>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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Subject: [9fans] FP register usage in Plan9 assembler
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I'm studying the 9front's amd64 kernel, and I'm pretty new to assembler
programming, so sorry if my question is too dumb...

I cannot understand the FP pseudo register usage.
The cpuid function, for example, is implemented as

/*
 * The CPUID instruction is always supported on the amd64.
 */
TEXT cpuid(SB), $-4
    MOVL    RARG, AX            /* function in AX */
    CPUID

    MOVQ    info+8(FP), BP
    MOVL    AX, 0(BP)
    MOVL    BX, 4(BP)
    MOVL    CX, 8(BP)
    MOVL    DX, 12(BP)
    RET

What I miss is where "info" comes from. I cannot

Apparently the GAS equivalent is:

.align 4
.globl cpuid
cpuid:
    mov    %ebp,%eax
    cpuid
    mov    0x10(%rsp),%rbp
    mov    %eax,0x0(%rbp)
    mov    %ebx,0x4(%rbp)
    mov    %ecx,0x8(%rbp)
    mov    %edx,0xc(%rbp)
    retq

Thus apparently info+8(FP) becomes 0x10(%rsp)
Why? I know that FP is a pseudo register, but shouldn't it be different
from SP?

And why info's value is 8? Is it the pointer size?

Another example:

TEXT insb(SB), 1, $-4
    MOVL    RARG, DX            /* MOVL    port+0(FP), DX */
    MOVQ    address+8(FP), DI
    MOVL    count+16(FP), CX
    CLD
    REP;    INSB
    RET

should be equivalent to

.align 4
.globl insb
insb:
    mov    %ebp,%edx
    mov    0x10(%rsp),%rdi
    mov    0x18(%rsp),%ecx
    cld
    rep insb
    retq

Again I cannot find a definition of address and count, but both seem to be
be valued as 8, why?


Giacomo

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<div dir=3D"ltr"><div><div>I&#39;m studying the 9front&#39;s amd64 kernel, =
and I&#39;m pretty new to assembler programming, so sorry if my question is=
 too dumb...<br><br></div>I cannot understand the FP pseudo register usage.=
<br></div>The cpuid function, for example, is implemented as <br><br><div s=
tyle=3D"margin-left:40px"><font face=3D"monospace,monospace">/*<br>=C2=A0* =
The CPUID instruction is always supported on the amd64.<br>=C2=A0*/<br>TEXT=
 cpuid(SB), $-4<br>=C2=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 RARG, AX=C2=A0=
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 /* function in AX */<br>=
=C2=A0=C2=A0=C2=A0 CPUID<br><br>=C2=A0=C2=A0=C2=A0 MOVQ=C2=A0=C2=A0=C2=A0 i=
nfo+8(FP), BP<br>=C2=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 AX, 0(BP)<br>=C2=
=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 BX, 4(BP)<br>=C2=A0=C2=A0=C2=A0 MOVL=
=C2=A0=C2=A0=C2=A0 CX, 8(BP)<br>=C2=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 D=
X, 12(BP)<br>=C2=A0=C2=A0=C2=A0 RET<br></font><br></div><div>What I miss is=
 where &quot;info&quot; comes from. I cannot <br><br></div><div>Apparently =
the GAS equivalent is:<br><br><div style=3D"margin-left:40px"><span style=
=3D"font-family:monospace,monospace">.align 4<br>.globl cpuid<br>cpuid:<br>=
=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 %ebp,%eax<br>=C2=A0=C2=A0=C2=A0 cp=
uid=C2=A0 <br>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 0x10(%rsp),%rbp<br>=
=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 %eax,0x0(%rbp)<br>=C2=A0=C2=A0=C2=
=A0 mov=C2=A0=C2=A0=C2=A0 %ebx,0x4(%rbp)<br>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=
=A0=C2=A0 %ecx,0x8(%rbp)<br>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 %edx,0=
xc(%rbp)<br>=C2=A0=C2=A0=C2=A0 retq=C2=A0=C2=A0 </span><br><br></div></div>=
<div>Thus apparently <font face=3D"monospace,monospace">info+8(FP)</font> b=
ecomes <span style=3D"font-family:monospace,monospace">0x10(%rsp)</span>=C2=
=A0 <br>Why? I know that FP is a pseudo register, but shouldn&#39;t it be d=
ifferent from SP? <br><br>And why <span style=3D"font-family:monospace,mono=
space">info</span>&#39;s value is 8? Is it the pointer size?<br><br></div><=
div>Another example:<br><br><div style=3D"margin-left:40px">TEXT insb(SB), =
1, $-4<br>=C2=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 RARG, DX=C2=A0=C2=A0=C2=
=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 /* MOVL=C2=A0=C2=A0=C2=A0 port+0(=
FP), DX */<br>=C2=A0=C2=A0=C2=A0 MOVQ=C2=A0=C2=A0=C2=A0 address+8(FP), DI<b=
r>=C2=A0=C2=A0=C2=A0 MOVL=C2=A0=C2=A0=C2=A0 count+16(FP), CX<br>=C2=A0=C2=
=A0=C2=A0 CLD<br>=C2=A0=C2=A0=C2=A0 REP;=C2=A0=C2=A0=C2=A0 INSB<br>=C2=A0=
=C2=A0=C2=A0 RET<br></div><br></div><div>should be equivalent to<br></div><=
div><br><div style=3D"margin-left:40px"><font face=3D"monospace,monospace">=
.align 4<br>.globl insb<br>insb:<br>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=
=A0 %ebp,%edx<br>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 0x10(%rsp),%rdi<b=
r>=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=C2=A0 0x18(%rsp),%ecx<br>=C2=A0=C2=A0=
=C2=A0 cld=C2=A0=C2=A0=C2=A0 <br>=C2=A0=C2=A0=C2=A0 rep insb<br>=C2=A0=C2=
=A0=C2=A0 retq</font><br></div><br></div><div>Again I cannot find a definit=
ion of <span style=3D"font-family:monospace,monospace">address </span>and <=
span style=3D"font-family:monospace,monospace">count</span>, but both seem =
to be be valued as 8, why?<br><br><br></div><div>Giacomo<br></div><div><br>=
</div></div>

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