Assembly x86 Understanding Define Word (DF) instruction -

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i'm starting play little bit assembly excuse me if questions dumb.

my questions (i'm trying divide 100 5, playing memory , variables of course!)

https://dl.dropboxusercontent.com/u/78049918/pics/capture.png

why dw (define word) instruction executed that? mean, understand of others line in particular dark 1 me..

also, can explain me why var instantiated @ address 0010ah ? think it's because 4 istructions before instantiation occupy 4*16bits=4*2bytes=8bytes, , var filed on stack right after. shouldn't instruction elsewhere? loaded on stack once program starts run?

bonus question: how can check what's on stack in emu8086?

because using assembler, not compiler. literally translating mnemonics , data flat binary file (or more commonly, headers included ie pe).

dw quite literally "declaring word". it's placing $0200 @ place in code. if look @ handy table, can see opcode 

add r8, r/m8

while i'm not going go whole opcode structure , disassembly, you'll see [r] meaning has modrm byte following opcode. here's handy table shows meaning of modrm byte. can see $00 [bx+si].

assembling whole instruction, 

add ax, [ax+si]

(we ax 0 in reg2 bits - ax first register indexed, cx 2nd, dx 3rd, bx 4th, etc). here's simple table docs in assembler:

modrm: 0 0 0 0 0 0 0 0 | | |   | |___| |_| |   |  reg1  mod |___|  (r/m)      reg2

ie: op reg2, reg1

this went little in depth, understand using assembler, not compiler, , little bit instruction encoding. things may little off (i'm used 32bit).

this reason executable formats have sections data , code, or in 16bit segments "isolated" each. in flat binary can have data section @ end or in blank areas jmp on it/between called functions.


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