Assembly Introduction

NOTE: This page is currently Under Construction =)



If your new to assembly, coming back to it, or refreshing and/or expanding your knowledge, a good place to begin is with the basics.

Notes

Before you begin, please take note of a few constants in this text. If we are talking about numbers, we may precede the number with (bin), (dec), (hex), which indicates a BINary, DECimal, or HEXadecimal value.

Assembly numbers

Assembly works with numbers in hexadecimal format. Hexadecimal is a base 16 numbering system. Values range from 0 to F; 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F. (e.g., (dec)15 = (hex)F, (dec)16 = (hex)10 - 1 'group' of 16, and no remainder. (dec)17 = (hex)11, (dec)31 = (hex)1F).

Registers

Registers are internal memory locations within the processor itself. Rather than forcing all variables to be loaded from RAM, processors provide registers which are small chucks of internal memory that can be accessed very easily, and usually without any performance penalties. The drawback of registers is that there are only a few of them. IA32 processors only provide 8 generic 32-bit registers. There are more, but they're there for specific purposes, and should not be used as generic storage. Registers are used mostly for local variables or caching frequently used data within a local function block or procedure; long term storage is handled in RAM, which we'll address later.

Generic IA-32 Registers

General purpose Registers


Flags

IA32 processors have a special register called EFLAGS. EFLAGS contains status and system flags. We're interested in the status flags, which will be discussed in depth later on.

Instruction Format

Instructions in IA-32 assembly generally consist of an opcode and 1 or more parameters, however some opcodes do not receive any parameters. Opcodes are operation codes, which give an instruction to the processor to preform a task.

Before we delve into all the various opcodes, it is a good idea to take a look at the aforementioned flags. As mentioned, flags contain system and status flags; we're interested in the status flags.

Arithmetic Flags

Overflow flags consist of CF (carry flag) and OF (overflow flag). There are 2 overflow flags to differentiate between signed and unsigned operations. Because signed integers are one bit smaller than their equivalent-sized unsigned counterparts (1 bit is used to hold the sign). OF represents overflows in signed operands and CF represents overflows in unsigned operands.
ZF (zero flag) represents an arithmetic operation where the result is zero.
SF (sign flag) receives the value of the most significant bit of the result of an arithmetic operation. For signed integers, this represents the sign of the result; 1 denotes negative, and 0 denotes positive (or zero).
PF (parity flag) is a rarely used flag that denotes the binary parity of the lower 8 bits of a given arithmetic result. Binary parity means that the flag reports the parity of the number of bits set, as opposed to the actual numeric parity of the result (i.e., 0's & 1's, expressed in binary to represent a decimal number; e.g., (bin)101 = (dec)5, and the parity flag is set to 0). 1 denotes an even number of set bits while 0 denotes an odd number of set bits in the lower 8 bits of the result.

Instuctions (Opcodes [operation codes])

Now it's time to look at some instructions.

* indicates 64-bit arithmetic operations. ** indicates integer conversion, from 8-bit to 16-bit or 32 bit; 16-bit to 32-bit. *** indicates converting a signed 32-bit integer to a signed 64-bit integer.
Conditional Code Constructs (Use bolded code to replace xx in Jxx above).

Signed Conditional Codes for CMP and SUB Instructions.
Note: Logical operators used in the EFLAGS are discussed below.

Unsigned Conditional Codes for CMP and SUB Instructions.

Logical Operators

Whew. All that and guess what... We haven't done and MASM yet !?!?!! Understanding the above principles is paramount to understanding the few abstractions made is masm32, so that you can read your code in a debugger. This page will be updated soon, so check back for more!