PROGRAM MEMORY-ADDRESSING MODES
Program memory-addressing modes, used with the JMP (jump) and CALL instructions, consist of three distinct forms: direct, relative, and indirect. This section introduces these three address- ing forms, using the JMP instruction to illustrate their operation.
Direct Program Memory Addressing
Direct program memory addressing is what many early microprocessors used for all jumps and calls. Direct program memory addressing is also used in high-level languages, such as the BASIC language GOTO and GOSUB instructions. The microprocessor uses this form of addressing, but not as often as relative and indirect program memory addressing are used.
The instructions for direct program memory addressing store the address with the opcode. For example, if a program jumps to memory location 10000H for the next instruction, the address (10000H) is stored following the opcode in the memory. Figure 3–14 shows the direct intersegment JMP instruction and the 4 bytes required to store the address 10000H. This JMP instruction loads CS with 1000H and IP with 0000H to jump to memory location 10000H for the next instruction. (An intersegment jump is a jump to any memory location within the entire memory system.) The direct jump is often called a far jump because it can jump to any memory location for the next
instruction. In the real mode, a far jump accesses any location within the first 1M byte of memory by changing both CS and IP. In protected mode operation, the far jump accesses a new code seg- ment descriptor from the descriptor table, allowing it to jump to any memory location in the entire 4G-byte address range in the 80386 through Core2 microprocessors.
In the 64-bit mode for the Pentium 4 and Core2, a jump or a call can be to any memory location in the system. The CS segment is still used, but not for the address of the jump or the call. The CS register contains a pointer to a descriptor that describes the access rights and privilege level of the code segment, but not the address of the jump or call.
The only other instruction that uses direct program addressing is the intersegment or far CALL instruction. Usually, the name of a memory address, called a label, refers to the location that is called or jumped to instead of the actual numeric address. When using a label with the CALL or JMP instruction, most assemblers select the best form of program addressing.
Relative Program Memory Addressing
Relative program memory addressing is not available in all early microprocessors, but it is avail- able to this family of microprocessors. The term relative means “relative to the instruction pointer (IP).” For example, if a JMP instruction skips the next 2 bytes of memory, the address in relation to the instruction pointer is a 2 that adds to the instruction pointer. This develops the address of the next program instruction. An example of the relative JMP instruction is shown in Figure 3–15. Notice that the JMP instruction is a 1-byte instruction, with a 1-byte or a 2-byte dis- placement that adds to the instruction pointer. A 1-byte displacement is used in short jumps, and a 2-byte displacement is used with near jumps and calls. Both types are considered to be intrasegment jumps. (An intrasegment jump is a jump anywhere within the current code segment.) In the 80386 and above, the displacement can also be a 32-bit value, allowing them to use relative addressing to any location within their 4G-byte code segments.
Relative JMP and CALL instructions contain either an 8-bit or a 16-bit signed displacement that allows a forward memory reference or a reverse memory reference. (The 80386 and above can have an 8-bit or 32-bit displacement.) All assemblers automatically calculate the distance for the dis- placement and select the proper 1-, 2- or 4-byte form. If the distance is too far for a 2-byte displace- ment in an 8086 through an 80286 microprocessor, some assemblers use the direct jump. An 8-bit displacement (short) has a jump range of between + 127 and – 128 bytes from the next instruction; a 16-bit displacement (near) has a range of ; 32K bytes. In the 80386 and above, a 32-bit displacement allows a range of ; 2G bytes. The 32-bit displacement can only be used in the protected mode.
Indirect Program Memory Addressing
The microprocessor allows several forms of program indirect memory addressing for the JMP and CALL instructions. Table 3–10 lists some acceptable program indirect jump instructions, which can use any 16-bit register (AX, BX, CX, DX, SP, BP, DI, or SI); any relative register ([BP], [BX], [DI], or [SI]); and any relative register with a displacement. In the 80386 and above, an extended register can also be used to hold the address or indirect address of a relative JMP or CALL. For example, the JMP EAX jumps to the location address by register EAX.
If a 16-bit register holds the address of a JMP instruction, the jump is near. For example, if the BX register contains 1000H and a JMP BX instruction executes, the microprocessor jumps to offset address 1000H in the current code segment.
If a relative register holds the address, the jump is also considered to be an indirect jump. For example, JMP [BX] refers to the memory location within the data segment at the offset address con- tained in BX. At this offset address is a l6-bit number that is used as the offset address in the intrasegment jump. This type of jump is sometimes called an indirect-indirect or double-indirect jump.
Figure 3–16 shows a jump table that is stored, beginning at memory location TABLE. This jump table is referenced by the short program of Example 3–14. In this example, the BX register is loaded with a 4 so, when it combines in the JMP TABLE[BX] instruction with TABLE, the effective address is the contents of the second entry in the 16-bit-wide jump table.
