diff options
Diffstat (limited to 'src/org')
| -rw-r--r-- | src/org/blog/assembly/1.org | 68 |
1 files changed, 44 insertions, 24 deletions
diff --git a/src/org/blog/assembly/1.org b/src/org/blog/assembly/1.org index daa4976..fa77e49 100644 --- a/src/org/blog/assembly/1.org +++ b/src/org/blog/assembly/1.org @@ -68,42 +68,62 @@ LOOP #+BEGIN_SRC asm MUL BX (DX, AX = AX * BX) #+END_SRC +** Addressing and registers...again +*** I realized what I wrote here before was almost gibberish, sooo here we go again I guess ? -*** Offset/Address Registers -*SP*: This is the stack pointer. It is of 16 bits. It points to the topmost item of the stack. If the stack is empty the stack pointer will be (FFFE)H (or 65534 in decimal). Its offset address is relative to the stack segment(SS). +Well lets take a step back to the notion of effective addresses VS relative ones. +*** Effective = 10h x Segment + Offset . Part1 +When trying to access a specific memory space, we use this annotation *[Segment:Offset]*, so for example, and assuming *DS = 0100h*. We want to write the value *0x0005* to the memory space defined by the physical address *1234h*, what do we do ? +**** Answer : +#+BEGIN_SRC asm +MOV [DS:0234h], 0x0005 +#+END_SRC -*BP*: This is the base pointer. It is of 16 bits. It is primarily used in accessing parameters passed by the stack. Its offset address is relative to the stack segment(SS). +Why ? Let's break it down : +[[../../../gifs/lain-dance.gif]] -*SI*: This is the source index register. It is of 16 bits. It is used in the pointer addressing of data and as a source in some string-related operations. Its offset is relative to the data segment(DS). -*DI*: This is the destination index register. It is of 16 bits. It is used in the pointer addressing of data and as a destination in some string-related operations. Its offset is relative to the extra segment(ES). -*** Segment Registers -*CS*: Code Segment, it defines the start of the program memory, and the different addresses of the different instructions relative to CS. +We Already know that *Effective = 10h x Segment + Offset*, So here we have : *1234h = 10h x DS + Offset*, we already know that *DS = 0100h*, we end up with this simple equation *1234h = 1000h + Offset*, therefor the Offset is *0234h* -*DS*: Data Segment, defines the start of the data memory where we store all data processed by the program. -*SS*: Stack Segment, or the start of the pile. The pile is a memory zone that is managed in a particular way, it's like a pile of plates, where we can only remove and add plates on top of the pile. Only one address register is enough to manage it, its the stack pointer SP. We say that this pile is a LIFO pile (Last IN, First OUT) +Simple, right ?, now for another example +*** Another example : +What if we now have this instruction ? +#+BEGIN_SRC asm + MOV [0234h], 0x0005 +#+END_SRC +What does it do ? You might or might not be surprised that it does the exact same thing as the other snipped of code, why though ? Because apparently and for some odd reason I don't know, the compiler Implicitly assumes that the segment used is the *DS* one. So if you don't specify a register( we will get to this later ), or a segment. Then the offset is considered an offset with a DS segment. + -*EX*: The start of an auxiliary segment for data -** The format of an address: -An Address must have this fellowing form [RS : RO] with the following possibilities: +*** Segment + Register <3 -- A value : Nothing -- ES : DI -- CS : SI -- ES : BP -- DS : BX +Consider *DS = 0100h* and *BX = BP = 0234h* and this code snippet: +#+BEGIN_SRC asm + MOV [BX], 0x0005 ; NOTE : ITS NOT THE SAME AS MOV BX, 0x0005. Refer to earlier paragraphs +#+END_SRC -*** Note 1 : -When the register isn't specified. the CPU adds it depending on the offset used : +Well you guessed it right, it also does the same thing, but now consider this : +#+BEGIN_SRC asm + MOV [BP], 0x0005 +#+END_SRC + +If you answered that its the same one, you are wrong. And this is because the segment used changes according to the offset as I said before in an implicit way. Here is the explicit equivalent of the two commands above: +#+BEGIN_SRC asm + MOV [DS:BX], 0x0005 + MOV [SS:BP], 0x0005 +#+END_SRC + +The General rule of thumb is as follows : - If the offset is : DI SI or BX, the Segment used is DS. -- If its BP, then the segment is SS. +- If its BP or SP, then the segment is SS. -*** Note 2 : -Apparently we will assume that we are in the DS segment and only access to memory using the offset. -*** Note 3 : -The values of the registers CS DS and SS are automatically initialized by the OS when launching the program. So these segments are implicit. AKA : If we want to access a specific data in memory, we just need to specify its offset. +**** Note +The values of the registers CS DS and SS are automatically initialized by the OS when launching the program. So these segments are implicit. AKA : If we want to access a specific data in memory, we just need to specify its offset. Also you can't write directly into the DS or CS segment registers, so something like +#+BEGIN_SRC asm +MOV DS, 0x0005 ; Is INVALID +MOV DS, AX ; This one is VALID +#+END_SRC |