Introduction
+
This section is intended
+to give one a general background in low-level programming, specifically
+related to video programming. It assumes that you already know how to program
+in your intended programming environment, and answers questions such as:
+
Why write hardware-level code?
+
One reason for writing hardware-level
+code is to develop drivers for operating systems or applications. Another
+reason is when existing drivers do not provide the required performance
+or capabilities for your application, such as for programming games or
+multimedia. Finally, the most important reason is enjoyment. There is a
+whole programming scene dedicated to producing "demos" of what the VGA/SVGA
+can do.
+
+
Do I need to know assembly language?
+
No, but it helps. Assembly
+language is the basis all CPU operations. All functions of the processor
+and computer are potentially accessible from assembly. With crafty use
+of assembly language, one can write software that exceeds greatly the potential
+of higher level languages. However, any programming environment that provides
+direct access to I/O and memory will work.
+
+
What are hex and binary numbers?
+
Humans use a number system
+using 10 different digits (0-9), probably because that is the number of
+fingers we have. Each digit represents part of a number with the rightmost
+digit being ones, the second to right being tens, then hundreds, thousands
+and so on. These represent the powers of 10 and is called "base 10" or
+"decimal."
+
Computers are digital (and
+don't usually have fingers) and use two states, either on or off to represent
+numbers. The off state is represented by 0 and the on state is represented
+by 1. Each digit (called a bit, short for Binary digIT) here represents
+a power of 2, such as ones, twos, fours, and doubling each subsequent digit.
+Thus this number system is called "base 2" or "binary."
+
Computer researchers realized
+that binary numbers are unwieldy for humans to deal with; for example,
+a 32-bit number would be represented in binary as 11101101010110100100010010001101.
+Converting decimal to binary or vice versa requires multiplication or division,
+something early computers performed very slowly, and researchers instead
+created a system where the numbers were broken into groups of 3 (such as
+11 101 101 010 110 100 100 010 010 001 101) and assigned a number from
+0-7 based on the triplet's decimal value. This number system is called
+"base 8" or "octal."
+
Computers deal with numbers
+in groups of bits usually a length that is a power of 2, for example, four
+bits is called a "nibble", eight bits is called a "byte", 16 bits is a
+"word", and 32 bits is a "double word." These powers of two are not equally
+divisible by 3, so as you see in the divided example a "double word" is
+represented by 10 complete octal digits plus two-thirds of an octal digit.
+It was then realized that by grouping bits into groups of four, a byte
+could be accurately represented by two digits. Because a group of four
+bits can represent 16 decimal numbers, they could not be represented by
+simply using 0-9, so they simply created more digits, or rather re-used
+the letters A-F (or a-f) to represent 10-15. So for example the rightmost
+digits of our example binary number is 1101, which translates to 13 decimal
+or D in this system, which is called "base 16" or "hexadecimal."
+
Computers nowadays usually
+speak decimal (multiplication and division is much faster now) but when
+it comes to low-level and hardware stuff, hexadecimal or binary is usually
+used instead. Programming environments require you to explicitly specify
+the base a number is in when overriding the default decimal. In most assembler
+packages this is accomplished by appending "h" for hex and "b" for binary
+to end of the number, such as 2A1Eh or 011001b. In addition, if the hex
+value starts with a letter, you have to add a "0" to the beginning of the
+number to allow it to distinguish it from a label or identifier, such as
+0BABEh. In C and many other languages the standard is to append "0x" to
+the beginning of the hex number and to append "%" to the beginning of a
+binary number. Consult your programming environment's documentation for
+how specifically to do this.
+
Historical Note: Another
+possible explanation for the popularity of the octal system is that early
+computers used 12 bit addressing instead of the 16 or 32 bit addressing
+currently used by modern processors. Four octal digits conveniently covers
+this range exactly, thus the historical architecture of early computers
+may have been the cause for octal's popularity.
+
+
What are the numerical conventions used
+in this reference?
+
Decimal is used often
+in this reference, as it is conventional to specify certain details about
+the VGA's operation in decimal. Decimal numbers have no letter after them.
+An example you might see would be a video mode described as 640x480z256.
+This means that the video mode has 640 pixels across by 480 pixels down
+with 256 possible simultaneous colors. Similarly an 80x25 text mode means
+that the text mode has 80 characters wide (columns) by 25 characters high
+(rows.) Binary is frequently used to specify fields within a particular
+register and also for bitmap patterns, and has a trailing letter b (such
+as 10011100b) to distinguish it from a decimal number containing only 0's
+and 1's. Octal you are not likely to encounter in this reference, but if
+used has a trailing letter o (such as 145o) to distinguish it from a decimal.
+Hexadecimal is always used for addressing, such as when describing I/O
+ports, memory offsets, or indexes. It is also often used in fields longer
+than 3 bits, although in some cases it is conventional to utilize decimal
+instead (for example in a hypothetical screen-width field.)
+
Note: Decimal numbers in
+the range 0-1 are also binary digits, and if only a single digit is present,
+the decimal and binary numbers are equivalent. Similarly, for octal the
+a single digit between 0-7 is equivalent to the decimal numbers in the
+same range. With hexadecimal, the single-digit numbers 0-9 are equivalent
+to decimal numbers 0-9. Under these circumstances, the number is often
+given as decimal where another format would be conventional, as the number
+is equivalent to the decimal value.
+
+
+
How do memory and I/O ports work?
+
80x86 machines have both
+a memory address space and an input/output (I/O) address space. Most of
+the memory is provided as system RAM on the motherboard and most of the
+I/O devices are provided by cards (although the motherboard does provide
+quite a bit of I/O capability, varying on the motherboard design.) Also
+some cards also provide memory. The VGA and SVGA display adapters provide
+memory in the form of video memory, and they also handle I/O addresses
+for controlling the display, so you must learn to deal with both. An adapter
+card could perform all of its functions using solely memory or I/O (and
+some do), but I/O is usually used because the decoding circuitry is simpler
+and memory is used when higher performance is required.
+
The original PC design was
+based upon the capabilities of the 8086/8088, which allowed for only 1
+MB of memory, of which a small range (64K) was allotted for graphics memory.
+Designers of high-resolution video cards needed to put more than 64K of
+memory on their video adapters to support higher resolution modes, and
+used a concept called "banking" which made the 64K available to the processor
+into a "window" which shows a 64K chunk of video memory at once. Later
+designs used multiple banks and other techniques to simplify programming.
+Since modern 32-bit processors have 4 gigabytes of address space, some
+designers allow you to map all of the video memory into a "linear frame
+buffer" allowing access to the entire video memory at once without having
+to change the current window pointer (which can be time consuming.) while
+still providing support for the windowed method.
+
Memory can be accessed most
+flexibly as it can be the source and/or target of almost every machine
+language instruction the CPU is capable of executing, as opposed to a very
+limited set of I/O instructions. I/O space is divided into 65536 addresses
+in the range 0-65535. Most I/O devices are configured to use a limited
+set of addresses that cannot conflict with another device. The primary
+instructions for accessing I/O are the assembly instructions "IN" and "OUT",
+simply enough. Most programming environments provide similarly named instructions,
+functions, or procedures for accessing these.
+
Memory can be a bit confusing
+though, because the CPU has two memory addressing modes, Real mode and
+Protected mode. Real mode was the only method available on the 8086 and
+is still the primary addressing mode used in DOS. Unfortunately, real mode
+only provides access to the first 1 MB of memory. Protected mode is used
+on the 80286 and up to allow access to more memory. (There are also other
+details such as protection, virtual memory, and other aspects not particularly
+applicable to this discussion.) Memory is accessed by the 80x86 processors
+using segments and offsets. Segments tell the memory management unit where
+in memory is located, and the offset is the displacement from that address.
+In real mode offsets are limited to 64K, because of the 16-bit nature of
+the 8086. In protected mode, segments can be any size up to the full address
+capability of the machine. Segments are accessed via special segment registers
+in the processor. In real mode, the segment address is shifted left four
+bits and added to the offset, allowing for a 20 bit address (20 bits =
+1 MB); in protected mode segments are offsets into a table in memory which
+tells where the segment is located. Your particular programming environment
+may create code for real and/or protected mode, and it is important to
+know which mode is being used. An added difficulty is the fact that protected
+mode provides for I/O and memory protection (hence protected mode), in
+order to allow multiple programs to share one processor and prevent them
+from corrupting other processes. This means that you may need to interact
+with the operating system to gain rights to access the hardware directly.
+If you write your own protected mode handler for DOS or are using a DOS
+extender, then this should be simple, but it is much more complicated under
+multi-tasking operating systems such as Windows or Linux.
+
+
How do I access these from my programming environment?
+
That is a very important
+question, one that is very difficult to answer without knowing all of the
+details of your programming environment. The documentation that accompanies
+your particular development environment is best place to look for this
+information, in particular the compiler, operating system, and/or the chip
+specifications for the platform.
+
+
Details for some common programming environments are given in: +