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Byte, Version 4.00 Copyright (c) 1987-89 SoftCircuits (tm) Program documentation Updated 8/6/89 Contents Page 1.0 Overview.....................................2 2.0 Getting Started..............................2 3.0 Manipulating the Byte Value..................3 4.0 The Command Window...........................5 5.0 Reading Memory...............................5 6.0 Program Release Information..................6 Appendix A.1 Bytes, Bits and Binary.......................6 A.2 What is ASCII?...............................7 A.3 What is Hexadecimal?.........................7 A.4 What is Octal................................7 A.5 How is the Color Attribute Used?.............8 A.6 Segmented Addressing.........................8 Page - 1 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits 1.0 Overview: This Program allows you to look at how a computer uses a byte of memory to store decimal, hexadecimal, octal, binary, ASCII, and color attribute values. This program is for anyone wanting a better understanding of how a series of tiny electronic switches, called bits, are used to store information. It will be of the most benefit to anyone learning to program in languages such as C and assembly. Byte can also serve as a reference for advanced programmers. This documentation describes all the features of Byte and also provides a brief tutorial in the appendix. 2.0 Getting Started: To start Byte, enter BYTE at the DOS prompt with BYTE.EXE in the current directory. 2.1 Options: You can include any of the following options when you start Byte: /q Quick display (CGA only). This option forces Byte to write to the screen as fast as possible. Use this option if you have a CGA display adapter that does not snow (flicker white spots). This option is used by default if your display adapter is not a CGA. /b Black and white. This option forces Byte to suppress colors. Use this option if you have a color adapter with a non-color display (composite monitors, for example). /m Suppress mouse support. This option forces Byte to ignore your mouse. It has no effect if you don't have a mouse. These options are entered on the command line when you start Byte. Note that multiple options must be separated with at least 1 space. The following example would start Byte in black and white mode and force it to ignore your mouse: byte /b /m Page - 2 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits 2.2 The Byte Screen: The Byte screen consists of eight connecting boxes that represent the eight bits of a byte. The boxes contain either a 1 to represent a bit that's turned on, or a 0 for a bit that's turned off. See appendix A.1 for more information about bytes and bits. The window in the lower portion of the screen is divided into 4 sections. The center section displays the value of the byte in decimal, hexadecimal, octal, binary, and ASCII notation. The top, right section displays the color attribute equivalent of the byte value. The section below that shows the current memory address. Section 5.0 shows how to read from, and change this address. The left section of the window is the command window. Section 4.0 describes the operations of this window. 2.3 The Menu: The top line of the screen shows the menu items currently available. You can select one by pressing the highlighted key. If you have a mouse, you can selected an item by clicking the mouse on the desired item. 3.0 Manipulating the Byte Value: This section describes the simplest methods to change the value of the byte. 3.1 Switching Bits: Byte allows you to make changes to the individual bits of the byte. 3.1.1 Moving the Bit Highlight: The bit highlight points to the bit that is to be affected by switching (explained in paragraph 3.1.2). You can use the left and right cursor arrows to move the highlight to any of the eight bits. Press the Home key to move the highlight to bit 0. This is the right-most bit. The End key will move the highlight to bit 7 (the left-most bit). 3.1.2 Switching Bits: Since the bits are either on or off, the bit boxes contain either a 1 to indicate on, or a 0 to indicate off. The cursor up key will switch the current bit on while the cursor down key switches it off. If you have a mouse, you can toggle a bit by clicking the left mouse button on the bit to be switched. See appendix A.1 for more information about bits. Page - 3 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits 3.2 Incrementing and Decrementing the Byte Value: A good way to get a feel for how a byte uses 8 bits to count is by incrementing the byte value and observing the action of the individual bits. To do this, press the PgUp key. You can hold this key down to watch Byte count rapidly. Likewise, the PgDn key causes Byte to count backwards, or decrement the byte value. 3.3 Reset and "Not": Pressing the Delete key will switch all of the bits to off. This makes the byte equal to 0. Pressing the Insert key will "Not" the bits. "Not" is a logical operator. In this instance, it simply switches every bit of the byte. If a bit is 0, it becomes 1, if it's 1 it becomes 0. 3.4 Shifting and Rotating Bits: Unless you've programmed in C or assembly language, you probably aren't familiar with shifting bits. But the idea is pretty straight forward. 3.4.1 Shifting: Pressing Shift-right (this means to press the right cursor key once while holding down the shift key) causes each bit to take on the value of the bit to its left, and the left most bit becomes zero. The result is that each bit seems to move to the right with the right-most bit dropping off at the end. Press Shift-left to reverse the direction. 3.4.2 Rotating: Pressing Control-right does the same thing as shifting right except that the right-most bit "wraps around" to the other side so that bit 7 takes on the value of bit 0. This results in the bits appearing to rotate around. Again, press Control-left to reverse the direction. Page - 4 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits 4.0 The Command Window: The most flexible way to change the byte value is by entering one of the following commands in the command window. MOV n Set the byte equal to n INC Increment (add 1) CLEAR Clear bits (byte = 0) DEC Decrement (subtract 1) ADD n Add n MUL n Multiply by n SUB n Subtract n DIV n Divide by n AND n And bits with n XOR n Xor bits with n OR n Or bits with n NOT Reverse each bit SHR n Shift right n bits ROR Rotate right n bits SHL n Shift left n bits ROL Rotate left n bits n can be any number from 0 to 255. You can specify decimal, hexadecimal, octal, or binary notation by appending d, h, o, or b to the number. n can also be an ASCII character enclosed in single or double quotation marks. Appending a d is optional because decimal is assumed by default. Examples: mov "a" This makes the byte value 97 (the ASCII value for "a"). add 3dh This adds 61 (hexadecimal 3D) to the value of the byte. Note that overflow is ignored. For example, if 1 is added to 255, the result would be 0. 5.0 Reading Memory: Byte makes it easy to read a value in your computer's memory. The current memory address is displayed in the lower, right portion of the screen. Any time this value is changed, the byte will be set to the value at the new memory location. 5.1 Entering a Memory Address: Press the F2 key to enter a new memory address. Addresses are entered in the form xxxx:xxxx. Where xxxx is a hexadecimal number from 0 - FFFF. The first number specifies the segment address, and the second number specifies the offset address. The two values are separated with a colon. If only one number is entered, it will specify the offset address. You can press Shift-F2 to re- read the current address. See appendix A.6 for more information about segmented addressing. Page - 5 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits 5.2 Adjusting a Memory Address: You can also increment or decrement the current address. This allows you to browse through an area of your computer's memory. Press Shift-Up and Shift-Down to increment and decrement the offset address. This has the effect of reading each consecutive byte. Press Shift-PgUp and Shift-PgDn to increment and decrement the segment address. This has the effect of reading every 16 bytes. 6.0 Program Release Information: The Byte package, which consists of the files BYTE.EXE and BYTE.DOC, may be used and distributed freely, on the condition that it is distributed in full and unchanged, and that no fee is charged for such use or distribution. This package is released as is, and SoftCircuits makes no expressed or implied warranties of any kind. SoftCircuits is always looking out for new ideas, as well as information about program errors (be sure to include the version number). If you have a comment or question, write us at the address below. All letters requesting a response, will receive one. SoftCircuits Programming Box 811 Tustin, CA 92681 - 0811 Appendix A.1 Bytes, Bits and Binary?: Since a computer is an electronic device, it's not able to work with characters, digits or conventional numbers. About all a computer "understands" is on or off. So, a system has been devised that uses thousands of tiny, electronic, on-off switches called bits to store information. For us to understand the computer's way of counting, we'll use a 1 to represent a bit that is turned on, and a 0 to represent a bit that's turned off. The Byte screen shows 8 such bits because there are, of course, 8 bits in one byte. If you were to ignore the boxes, all you would have left is a row of zeros and ones. This row of zeros and ones represents the binary numbering system. Binary because each digit only has two possible values (0 or 1) as opposed to our traditional numbering system (decimal) in which each digit has 10 possible values (0 - 9). So we can say that 00000000b is how we write 0 in binary notation. The b is added to indicates that it's a binary number. Page - 6 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits For us to be able to store as many different numbers as possible in one byte, we need to assign each bit a different value. You may have noticed the bit numbers shown on the Byte display, 0 - 7 from right to left. Well, the value that we'll give to each bit will be 2 to the power of its bit number. For example, bit 3, when turned on, has a value of 2 to the power of 3. It adds 8 (2 * 2 * 2) to the total value of the byte. To illustrate, activate Byte and type mov 1 in the command window and press Return. The bits are now 00000001b. Notice that the decimal value, in the lower portion of the screen is 1 (2 to the power of 0). Next, enter shl 1 or press Shift-left. This moves the bit that is switched on to bit 1. Now the bits are 00000010b. Notice that the decimal value is now 2 (2 to the power of 1). You can continue shifting the bits and observe the decimal value change each time you do. Next, press the Delete key. The byte now equals 0. Now press the PgUp key repeatedly. This increments the value of the byte. Notice both the decimal and bit values as Byte counts. A.2 What is ASCII?: ASCII is the acronym for American national Standard Code for Information Interchange. The computer stores a table of ASCII characters in memory, and when the computer is instructed to write a character to the screen, it looks up the value in the table and then displays the character that corresponds to that value. A.3 What is Hexadecimal?: Where decimal is base 10 and binary is base 2, hexadecimal is base 16. Since 16 digits are needed, the hexadecimal digits are 0 - 9, followed by A - F. Hexadecimal notation is used often in computers because a hex digit has exactly the same range as 4 bits (one nibble). Two hex digits have exactly the same range as 8 bits (one byte). You can easily become familiar with the hexadecimal numbering system by observing the hexadecimal display on the Byte screen while changes are made to the byte value. A.4 What is Octal?: Where decimal is base 10 and hexadecimal is base 16, octal is base 8. Page - 7 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits A.5 How is the Color Attribute Used?: When an IBM-standard personal computer is in normal text mode, it displays 2000 characters on the screen (25 x 80). The computer uses two bytes of memory for each character. One byte contains the ASCII value (character), and the other byte contains the character's color attribute. The box in the lower right portion of the Byte screen shows the color attribute for the current byte value. You will, of course, see more variation if you're using a color monitor. This box may be confusing at first if you're used to languages like BASIC, but if you ever want to use color in a program written in assembly language or C, then you will want a good understanding of the relationship between the byte value and the colors you see in the color attribute window. A common type of color monitor is the RGB monitor. So named because the screen is made up of Red, Green and Blue dots. It can be interesting and informative to watch the color attribute change as different bits are switched on and off. The following table shows the meaning of each bit in relationship to the color attribute. Bit: Meaning: 0.............Blue(foreground) 1............Green(foreground) 2..............Red(foreground) 3...........Bright(foreground) 4.............Blue(background) 5............Green(background) 6..............Red(background) 7............Blink(foreground) As already stated, monochrome monitors will, of course, show less variation than color monitors. But in text mode, they use video memory in exactly the same way (2000 characters, one byte for ASCII, one byte for the color attribute). Bits 3 and 7 have the same effect on both types of monitors. On a monochrome monitor, bit 0 by itself, shows underlined characters. Underlined characters aren't available on color monitors. A.6 Segmented Addressing: The central processing unit (CPU) of the IBM-standard personal computer works with numbers in word units. A word is equal to 16 bits, and 16 bits are equal to 2 bytes. So, a word has a range of 0 - 65,535 (hexadecimal FFFF). This range is often referred to as 64k. However, you're probably aware that IBM-standard personal computers are capable of accessing considerably more than 64k. So how does the computer use word values to keep track of more than 64k of memory? This brings us to segmented addressing, one of the most confusing aspects about IBM-standard personal computers. Page - 8 of 9 Byte, Version 4.00, Copyright (c) 1987-89 SoftCircuits Segmented addressing is the method used to address more than 64k using word values. This method has an effective range of 0 - 1,048,575 (hexadecimal FFFFF). This range is often referred to as 1 meg. Segmented addressing is written in the form: xxxx:xxxx Where xxxx is a hexadecimal number from 0 - FFFF. The first number specifies the segment address and the second number specifies the offset address. The two numbers are combined to create a single address value by shifting the segment 4 bits left and adding the two together. Segment: B377 Offset: 48D0 ===== Address: B8040 So B377:48D0 points to address B8040 (decimal 753,728). One peculiarity that results is that the same address can be written different ways. The following addresses would all point to the same location in memory. B800:0040 = B8040 B804:0000 = B8040 B000:8040 = B8040 B377:48D0 = B8040 You may wonder why the numbers weren't combined as follows: B377:48D0 = B37748D0 This is the way programs store variables too large to fit in a single word. This would give the computer a range of 0 - 4,294,967,295! However, this method would mean that each time the computer executed an instruction, it would have to recalculate both the segment and offset values to determine the address of the next instruction. This would be much less efficient than the method used. Page - 9 of 9