The first 256 ports are directly addressable (address in the instruction) by some input/ output instructions, other instructions let you address the total64K ports indirectly (address in a register).
REGISTER STRUCTURE
The 8088 processor contains the thirteen 16- bit registers and nine I-bit flags shown in Figure 2-2. Notice that the thirteen registers are divided into three files of four registers each plus the thirteenth register, namely the instruction pointer (IP) (called the program counter in earlier processors). The IP is not directly accessible to the. programmer; it is manipulated with control-transfer instructions.
Data Register File
The data registers (top file Fig. 2-2) can be addressed as either 8- or 16-bit registers. (Note vertical line showing byte divisions).
150
I LO GICAL ADDRESS IADDRESSOFFSET
15
| EGMENT REGISTE | 1-....1....__....1 | SEGMENT |
| ADDRESS |
| |
I1920-BITGI
PHYSICAL MEMORY ADDRESS
Figure 2-1. How to Address One Million Bytes
The data registers handle both byte and word quantities with equal ease. Figure 2-2 shows that the 16-bit registers are named AX, BX, CX, and DX; and the 8-bit registers are named AL, AH, BL, BH, CL, CH, DL, and DH (the Lor H suffix designates high-order or low-order byte).
Generally, the data registers participate inter- changeably in both arithmetic and logical operations of the 8088. However, some instructions (e.g. string instructions) require certain general registers for specific uses. Fig- ure 2-3 shows which registers are implicitly used for special operations. Notice how Fig- ure 2-3 relates to Figure 2-2.
To review, data registers may be addressed as either 8-bit or 16-bit registers as shown in Figure 2-2. The registers in the next 2 files are addressed only as 16-bit registers.
Pointer and Index Register File
The pointer and index registers of the 8088 consist ofthe 16-bit registers SP, BP, SI, and DI as shown in Figure 2-2. These registers usually contain offset addresses for address- ing within a segment. They reduce program size by eliminating the need for each instruc- tion to specify frequently used addresses. These registers serve another (and perhaps more important) function; they provide for dynamic logical address computation as des- cribed in the section on operand addressing below. To accomplish this, the pointer and index registers participate in arithmetic and logical operations along with the 16-bit data registers described above.
Figure 2-2 shows this file divided into the pointer subfile (SP and BP) and the index subfile (SI and DI). The pointer registers provide convenient access to the current stack segment (as opposed to the data seg- ment). Unless otherwise specified in the instruction, pointer registers refer to the cur- rent stack segment while index registers refer to the current data segment.
