Real vs Protected Mode Addressing

• To form a physical memory address, appropriate segment register contents are required.

Key Memory Values

• Base = 23000000, Limit = 012FFH and G = 1
• Resulting in a value of 242FFFFFH
• 0CD1:02E0 results in OCFFOH
• 1000H as a segment register value correlates to addresses "10000, 1FFFF"

Operating Mode

• The DOS operating system requires the microprocessor to operate in real mode.
• The 80286 could be operated in protected mode but only has a 24-bit address bus.
• In real mode, memory exists at locations 00000H - FFFFFH.
• In real mode, a memory segment that begins at 10000 will end at 1FFFFH.
• In protected mode, the 80286 – Core processors' memory exists at locations 000000 - FFFFFFH.

Segment Access and Addressing

• TI = 0 indicates a global descriptor table.
• Segment: offset address combinations point to a common physical address such as A1B2:000C, A1B0:002C, and A1B1:001C.
• In protected mode, segment access is allowed if RPL >= DPL
• Segment registers should only contain segment addresses.
• A selector value in protected mode gives segment information, excluding the ending address.
• Memory locations 00390H through 00393H contain 9A, 76, 65, and 1F.
• With DS=0030H, SI=0090H, BP=0002H, then:
  • MOV AX, [SI] = 00390 = 9A
  • MOV AX, [SI+1] = 00391 = 76
  • MOV AX, [SI][BP] = 00392 = 65

Instructions and Registers

• If SI = 1000H, MOV[SI], BH writes a byte of data from BH into memory location 1000H.
• Instruction MOV DS:[2000H], AL copies byte size data in AX to the memory.
• With CS = 2000 and IP = 1000H, the result is 21000H.
• DS=B9DC, SS=F7CA, AX=DC5, DI=8E9, BP=2CE, SI=97D, Var1=12A, AX=3DF9 and Var2=D32; MOV [BP+DI], AX.
• Adding BP and DI, then adding it onto SS, then adding to AX: answer is F8857H (considering bits of registers BP & DI).
• The first 1Mbyte of memory is called the real and conventional memory system.

Selectors and Descriptors

• The selector selects the descriptor from a descriptor table and selects any location within the 64k byte memory segment.
• A descriptor describes all in the memory segment except boundaries.

Addressing Modes

• Immediate Addressing Mode directly specifies the operand value.
• Offset Address are sometimes called displacements or relative.
• Segment Register should be appended with zero.
• Ending address of segment located by segment register value D6B2H is D6B20+FFFF = E6B1F.
• Ending address of segment located by segment register value E44AH is F449F.
• The addressing mode where you directly specify the operand value is the Immediate Addressing Mode.

Jumps

• Intersegment Jump is a jump to any memory location within the entire memory system.
• For CS=6900; JMP[76F3CH], located at 5E333H is the new value of IP
• After Far jump, IP = bytes 2 and 3 of the instruction with a value close to 4F3C/DF3C
• CS = D82AH; JMP [5F] located at DF78 AB25H is a short jump (CS + (Displacement + IP) D82FF

Memory Protection

• In protected mode, memory protection is implemented by restricting access to memory segments through privilege levels and access rights.

Memory Addressing Calculations

• With DS = 1000H, SS = 2000, BP = 1000, DI = 0100; MOV AL, [BP+DI] results in -21100H
• ADD instruction stores a value 1308h at offset value 9F71h, computed address 7CB3H requires (Computed address-Offset) + FFFF or 82BBFFH.
• DS=B9DC, SS=F7CA, ES=AB32, AX=DC5, DI=8E9, BX=D98, BP=2CE, Var1=12A and Var2=D32; MOV CX, [SI+DEE]
• When calculating A9A8B; since CX and destination 16-bit ang content, then A9A8C; AB is where the higher content which is hinahanap kasi nga 16-bit
• Effective address of the memory location is visibly part of the Direct Mode instruction.

Indirect Addressing

• In stack memory-addressing modes, the low-order 8-bits are placed in the location addressed by SP-2
• The instruction tranfers a byte or word between a register and a memory location addressed by an index or base register (Register Indirect addressing mode).
• moves a byte or a word between a register and the memory location addressed by an index or base register plus a displacement = Register Relative Addressing Mode
• transfers a byte or a word between a register and the memory location addressed by a base register (BP or BX) plus an index register (DI or SI) = Base-Plus-Index Addressing Mode
• should be added to the starting address to get the ending address = FFFFH
• MOV AX, [BX], is an example ofRegister indirect addressing mode Contains the location where the next instruction in the code segment should be fetched = IP
• TM=036H, EBP=106B C443H, ESI=A34C 6A91, EDI=8BED C437H; DS=0044H; MOV TM [EBP][EDI+ECH], SI = 9C598E5C
• Var=B387, ECX=9612 E846, ESI=497C D1B4, EDI=4C97 B528, EBX=BEB8 4BE5H, DS=0036; MOV EBP, Var3[ECX][EDI-A981] = E2AB55455

Memory Capacity

• 32-bit microprocessors in protected mode can address up to 4GB of memory.

Explanation of physical memory addressing using segment registers. Explores real mode (DOS) and protected mode. Includes key memory values and operating mode considerations for microprocessors like the 80286 and core processors.