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madfrogme

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Memory Operands.
In Intel syntax the base register is enclosed in ’[’ and ’]’ where as in AT&T they change to ’(’ and ’)’. Additionally, in Intel syntax an indirect memory reference is like

section:[base + index*scale + disp], which changes to

section:disp(base, index, scale) in AT&T.

One point to bear in mind is that, when a constant is used for disp/scale, ’$’ shouldn’t be prefixed.

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2012-10-02 01:55

madfrogme

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Protected Mode and Real Mode each use a different format for the interrupt table — PM uses an 8-byte entry per interrupt and RM uses a 4-byte entry.

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2012-10-02 12:39

madfrogme

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sidt 语句

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2012-10-02 15:24

madfrogme

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In some cases, a single variable may serve as both the input and the output operand.

asm( "incl %0" : "=a"(var): "0"(var));

The register %eax is used as both the input and the output variable.

var input is read to %eax and updated %eax is stored in var again after increment.

"0" here specifies the same constraint as the 0th output variable

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2012-10-02 16:26

madfrogme

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out 语句
Usage: OUT port,accum

Modifies Flags: None

Transfers byte in AL,word in AX or dword in EAX to the specified hardware port address.

If the port number is in the range of 0-255 it can be specified as an immediate.

If greater than 255 then the port number must be specified in DX.

Since the PC only decodes 10 bits of the port address, values over 1023 can only be decoded by third party vendor equipment and also map to the port range 0-1023.

[ 本帖最后由 madfrogme 于 2012-10-2 17:36 编辑 ]

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2012-10-02 16:35

madfrogme

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If BIOS is to recognize a device as a bootable device,

then the device should have the values 0x55 and 0xaa at the 510th and 511th location.

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2012-10-02 21:04

madfrogme

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LODSB loads a byte from [DS:SI] or [DS:ESI] into AL. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) SI or ESI.

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2012-10-03 20:16

madfrogme

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第 58 楼

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pushl %ebx     # equivalent instructions
                                    subl $4,  %esp
                                    movl %ebx, (%esp)

popl %ebx             # equivalent instructions
                                movl (%esp), %ebx
                                addl   $4, %esp

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2012-10-03 23:23

madfrogme

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MOV [SI], AX.

Will write the word contained in ax to the address DS:SI

MOV ES:[DI], AX.

Will write the word contained in ax to the address es:di

COMSB

will compare the byte at DS:SI to the byte at ES:DI, set the zero flag if they are equal and decrement /increment SI and DI according to the state of the directing flag

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2012-10-04 11:47

madfrogme

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The interrupt gate is used to specify an interrupt service routine.

When you do INT 50 in assembly, running in protected mode,

the CPU looks up the 50th entry(located at 50*8) in the IDT.

Then the interrupt gates selector and offset value is loaded.

The selector and offset is used to call the interrupt service routine.

When the IRET instruction is read , it returns.

[ 本帖最后由 madfrogme 于 2012-10-5 00:04 编辑 ]

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2012-10-04 13:31

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