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[Qemu-discuss] Experimenting with x86 real mode code in qemu

From: Jay Aurabind
Subject: [Qemu-discuss] Experimenting with x86 real mode code in qemu
Date: Mon, 7 Dec 2015 20:47:40 +0530

Hello Everyone,

I am doing an experiment to emulate x86 real mode in qemu so as to
study assembly programming in GNU assembler. My current status is that
qemu exits with the error:

qemu: fatal: Trying to execute code outside RAM or ROM at 0x000a0000

Please help me get my expt working. Coming to the details, my simple
source files:

.globl start

    jmp main

.globl main
main:   mov     $1, %ax
        mov     $1, %di
        mov     $1, %si
        mov     $1, %dx

Linker script: As x86 has the hard coded value 0xFFFF0 for the address
of first execution, I am putting my startup script which calls the
main function at 0xFFFF0:

    .text : { *(.text) }
    .data : { *(.data) }
    .bss : { *(.bss COMMON) }
    . = 0xFFFF0;
    .startup . : { startup.o (.text)}


My make output:

cc -nostdinc -nostdlib -ffreestanding -nostartfiles -nodefaultlibs
-mno-red-zone -c startup.S -o startup.o
cc -nostdinc -nostdlib -ffreestanding -nostartfiles -nodefaultlibs
-mno-red-zone -c main.S -o main.o
ld -T link.ld startup.o main.o -o bin

Later creating flash image:
dd if=/dev/zero of=flash.bin bs=4096 count=1024
dd if=bin of=flash.bin bs=4096 conv=notrunc

And finally starting qemu:
qemu-system-i386  -pflash flash.bin -nographic

That fails with the error mentioned I already mentioned :
WARNING: Image format was not specified for 'flash.bin' and probing guessed raw.
         Automatically detecting the format is dangerous for raw
images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.
qemu: fatal: Trying to execute code outside RAM or ROM at 0x000a0000

EAX=00000000 EBX=00000000 ECX=00000000 EDX=00000663
ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
EIP=000afff2 EFL=00000046 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0000 00000000 0000ffff 00009300
CS =f000 ffff0000 0000ffff 00009b00
SS =0000 00000000 0000ffff 00009300
DS =0000 00000000 0000ffff 00009300
FS =0000 00000000 0000ffff 00009300
GS =0000 00000000 0000ffff 00009300
LDT=0000 00000000 0000ffff 00008200
TR =0000 00000000 0000ffff 00008b00
GDT=     00000000 0000ffff
IDT=     00000000 0000ffff
CR0=60000010 CR2=00000000 CR3=00000000 CR4=00000000
DR0=00000000 DR1=00000000 DR2=00000000 DR3=00000000
DR6=ffff0ff0 DR7=00000400
CCS=00000000 CCD=00000000 CCO=ADDB
FCW=037f FSW=0000 [ST=0] FTW=00 MXCSR=00001f80
FPR0=0000000000000000 0000 FPR1=0000000000000000 0000
FPR2=0000000000000000 0000 FPR3=0000000000000000 0000
FPR4=0000000000000000 0000 FPR5=0000000000000000 0000
FPR6=0000000000000000 0000 FPR7=0000000000000000 0000
XMM00=00000000000000000000000000000000 XMM01=00000000000000000000000000000000
XMM02=00000000000000000000000000000000 XMM03=00000000000000000000000000000000
XMM04=00000000000000000000000000000000 XMM05=00000000000000000000000000000000
XMM06=00000000000000000000000000000000 XMM07=00000000000000000000000000000000


Please let me know what I missed.

Thanking in Advance!



Thanks and Regards,
Aurabindo J

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