Memory Access(8086 Assembler Tutorial for Beginners)

Memory Access
To access memory we can use these four registers: BX, SI, DI, BP. Combining these registers inside [ ] symbols, we can get different memory locations. These combinations are supported (addressing modes)

d8 - stays for 8 bit displacement.

d16 - stays for 16 bit displacement.

Displacement can be a immediate value or offset of a variable, or even both. It's up to compiler to calculate a single immediate value.
Displacement can be inside or outside of [ ] symbols, compiler generates the same machine code for both ways.
Displacement is a signed value, so it can be both positive or negative.

Memory access

Generally the compiler takes care about difference between d8 and

d16, and generates the required machine code. For example, let's assume that DS = 100, BX = 30, SI = 70. The following addressing mode: [BX + SI] + 25 is calculated by processor to this physical address: 100 * 16 + 30 + 70
+ 25 = 1725. By default DS segment register is used for all modes except those with BP register, for these SS segment register is used. There is an easy way to remember all those possible combinations using this chart:

You can form all valid combinations by taking only one item from each column or skipping the column by not taking anything from it. As you see BX and BP never go together. SI and DI also don't go together. Here is an example of a valid addressing mode: [BX+5].

      

                                                                     

You can form all valid combinations by taking only one item from each column or skipping the column by not taking anything from it. As you see BX and BP never go together. SI and DI also don't go together. Here is an example of a valid addressing mode: [BX+5].

In order to say the compiler about data type, these prefixes should be used:
BYTE PTR - for byte.
WORD PTR - for word (two bytes). For example:
BYTE PTR [BX] ; byte access.
or
WORD PTR [BX] ; word access.
MicroAsm supports shorter prefixes as well:
b. - for BYTE PTR
w. - for WORD PTR Sometimes compiler can calculate the data type automatically, but you may not and should not rely on that when one of the operands is an immediate value.

MOV instruction
· Copies the second operand (source) to the first operand (destination).
· The source operand can be an immediate value, general-purpose register or memory location.
· The destination register can be a general-purpose register, or memory location.
· Both operands must be the same size, which can be a byte or a word.




These types of operands are supported:
MOV REG, memory 
MOV memory, REG
MOV REG, REG 
MOV memory, immediate 
MOV REG, immediate
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...
immediate: 5, -24, 3Fh, 10001101b, etc...

For segment registers only these types of MOV are supported:

MOV SREG, memory 

MOV memory, SREG 

MOV REG, SREG 

MOV SREG, REG
SREG: DS, ES, SS, and only as second operand: CS.
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...

The MOV instruction cannot be used to set the value of the CS and  IP registers

.

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8086 Assembler Tutorial for Beginners

8086 Assembler Tutorial for Beginners(PART-1)


 This tutorial is intended for those who are not familiar with assembler at all, or have a very distant idea about it. Of course if you have knowledge of some other programming language (Basic, C/C++, Pascal...) that may help you a lot. But even if you are familiar with assembler, it is still a good idea to look through this document in order to study emu8086 syntax. It is assumed that you have some knowledge about number representation (HEX/BIN), if not it is highly recommended to study
Numbering Systems Tutorial before you proceed.

What is an assembly language?
Assembly language is a low level programming language. You need to get some knowledge about computer structure in order to understand anything. The simple computer model as I see it: 

The system bus (shown in yellow) connects the various components of a computer. 

The CPU is the heart of the computer, most of computations occur inside the CPU.


RAM is a place to where the programs are loaded in order to be executed.

GENERAL PURPOSE REGISTERS 8086 CPU has 8 general purpose registers, each register has its own name:
· AX - the accumulator register (divided into AH / AL).
· BX - the base address register (divided into BH / BL).
· CX - the count register (divided into CH / CL).
· DX - the data register (divided into DH / DL).
· SI - source index register.
· DI - destination index register.
· BP - base pointer.
· SP - stack pointer. Despite the name of a register, it's the programmer who determines the usage for each general purpose register. The main purpose of a register is to keep a number (variable). The size of the above registers is 16 bit, it's something like: 0011000000111001b (in binary form), or
12345 in decimal (human) form. 4 general purpose registers (AX, BX, CX, DX) are made of two separate 8 bit registers, for example if AX= 0011000000111001b, then AH=00110000b and AL=00111001b. Therefore, when you modify any of the 8 bit registers 16 bit register is also updated, and vice-versa. The same is for other 3 registers, "H" is for high and "L" is for low part. Because registers are located inside the CPU, they are much faster than memory. Accessing a memory location requires the use of a system bus, so it takes much longer. Accessing data in a register usually takes no time. Therefore, you should try to keep variables in the registers. Register sets are very small and most registers have special purposes which limit their use as variables, but they are still an excellent place to store temporary data of calculations.

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