The 80x86 IBM PC and Compatible Computers- P8: Praised by experts for its clarity and topical breadth, this visually appealing, one-stop source on PCs uses an easy-to-understand, step-by-step approach to teaching the fundamentals of 80x86 assembly language programming and PC architecture. Offering users a fun, hands-on learning experience, it uses the Debug utility to show what action the instruction performs, then provides a sample program to show its application. | Example 11-3 Show the design of IN AL 9FH using the 74LS244 as a tri-state buffer. Solution 9FH is decoded then ANDed with IOR. To activate OC of the 74LS244 it must be inverted since OC is an active-low pin. This is shown in Figure 11-5. to DO - D7 of system data bus 1. Designers use a __ latch tri-state buffer for output and a _ latch tri-state buffer for input. 2. Why do we use latches in I O design 3. Why is the 74LS373 called the transparent latch 4. To use the 73LS373 as a latch OC must be set topermanently. 5. True or false. To access the maximum number of ports in the x86 we must decode addresses AO - Al5. 6. In memory-mapped I O which signal is used to select the a output and b input devices this watermark. 315 SECTION I O ADDRESS MAP OF X86 PCS Designers of the original IBM PC decided to make full use of I O instructions. This led to assignment of different port addresses to various peripherals such as LPT and COM ports and other chips and devices. The list of the designated I O port addresses is referred to as the I O map. Table 11-1 shows the I O map for the x86 PC. A much more detailed I O map of the x86 PC is given in Appendix G. Any system that needs to be compatible with the x86 IBM PC must follow the I O map of Table 11-1. For example the map shows that we can use I O address 300 - 3IF for a prototype card. This is shown below. Absolute vs. linear select address decoding In decoding addresses either all of them or a selected number of them are decoded. If all the address lines are decoded it is called absolute decoding. If only selected address pins are used for decoding it is called linear select decoding. Linear select is cheaper since the less input there is the fewer the gates needed for decoding. The disadvantage is that it creates what are called aliases the same port with multiple addresses. In cases where linear select is used we must document port addresses in the I O map thoroughly. In the first IBM PC .
