Addressing Modes and Memory Segmentation

• In stack memory addressing modes, the low-order 8 bits are placed in the location addressed by SP.
• Immediate Mode is the addressing mode where the operand value is directly specified.
• The instruction MOV AX, 2020H uses Immediate Addressing Mode and moves 16 bits of information.
• A descriptor describes the location, size, boundaries, and access rights of a memory segment.
• An intersegment jump is a jump to any memory location within the entire memory system.
• With DS = 1A00H and ECX = 00002000H, the memory location addressed in real mode Core2 is 0001C000H.
• With DS = 1000H, SS = 2000H, BP = 1000H, and DI = 0100H, MOV AL,[BP+DI] accesses memory address 21100H in real mode.
• Direct Mode is an addressing mode where the effective address of the memory location is visibly part of the instruction.
• "MOV DX, [BX+04]" and "MOV AX, [BX]" are examples of register Indirect Addressing Mode.
• If memory locations 00390H through 00393H contain 9A, 76, 65, and 1F respectively, and DS = 0030H and SI = 0090H and BP = 0002H, then for MOV AX,[SI+1], AX contains 769AH, for MOV AX,[SI][BP] AX contains 1F65H.
• The selector selects the descriptor from a descriptor table.
• With CS = 2000H and IP = 1000H, the memory address of the next instruction executed by the microprocessor is 21000H.
• A memory segment that begins at AB208000H and ends at AC20AFFFH, is a writable 32-bit data segment growing upward with privilege level set at 2nd highest and not accessed, is coded with values 02, 10, 00, 80, 20, B2, D0, and AB.
• If a descriptor has Base = 23000000H, Limit = 012FFH, and G = 1, the described segment ends at 242FFFFFH.
• In Real Mode, a memory segment that begins at 10000H will end at 1FFFFH.
• The DOS operating system requires the microprocessor to operate in real mode.
• In protected mode memory, protection is implemented by restricting access to memory segments using privilege levels.

Descriptor Tables

• When TI=0, TI is A global descriptor table.

Memory Segments

• The offset address selects any location within the 64K byte memory segment.
• 32-bit microprocessors operating in protected mode can address up to 4GB of memory.
• To form a physical memory address, segment register contents are added to 16-bit offset address shifted left by 4 positions and operated using one of the addressing modes.
• If CS contains 2001H and IP contains 007CH, the physical memory location accessed for the next instruction is 2008CH.
• For the 80286-Core processors, the protected mode memory exists at locations 000000H-FFFFFFH.
• The instruction MOV AX,[BX] is an example of register indirect addressing mode.
• The physical address accessed by segment:offset combination A359:B3FD is AE98D.
• The IP register contains the location where the next instruction in the code segment should be fetched.
• The 80286 could be operated in protected mode but with only a 24-bit address bus.
• If SI = 1000H, then MOV [SI], BH writes a byte of data from BH into memory location 1000H.
• The instruction MOV DS:[2000H], AL is illegal and not allowed.
• If the segment register value is 1000H, the starting address of the segment is 10000H and the ending address is 1FFFFH.
• Segments offset combinations that point to A common physical memory address are a) A1B2:000C, c) A1B0:002C, and d) A1B1:001C.
• In protected mode, a selector value gives the following segment information EXCEPT the ending address.
• The first 1-Mbyte of memory is called both conventional and real mode.
• The segment registers must never be used as data registers to hold arbitrary values as they should only contain segment addresses.
• The instruction MOV AX, 1234H is an example of immediate addressing mode.
• Real mode memory exists at locations 00000H-FFFFFH.
• Indirect addressing mode uses a register to hold the actual address that identifies either the source or the destination to be used in the data transfer.
• In real mode, the physical address accessed by the segment-offset combination 0CD1:02E0 is OCFF0H.
• In protected mode, access to the segment is allowed if RPL = DPL.
• The instruction MOV AX, [BX] is an example of register indirect addressing mode.
• Register Relative moves a byte or word between a memory location and a register.
• Register Relative Addressing moves a byte or word between a register and memory location addressed by an index or base register plus displacement.
• Based Index Addressing moves a byte or word between a register and memory location addressed by a base register (BP or BX) plus an index register (DI or SI).
• To get the ending address, the offset should be added to the starting address.
• If an ADD instruction stores a value 7308H at offset value 9F71H, and the computed address is 7CB31H, assuming real mode, the starting address is 72BC0.
• If TM = 036H, EBP = 106B C443H, ESI = A34C 6181H, EDI = 8BED C437H, and DS = 0044H, then the address/es accessed by the instruction MOV TM[EBP][EDI+ECH], SI, assuming protected mode is D2D69315.
• If DS = B9DC, SS = F7CA, ES = AB32, AX = DC5, DI = 8E9, BX = D98, BP = 2CE, SI = 97D, Var1=12A and Var2=D32, after executing the instruction MOV CX, the content of the destination is 4FDCH.