History

• 8086 designed to mimic 8080 for porting ease
• 8080 had 16 bits (64k) of addressing, 8 bits per address for up to 64k of memory. (You put 16 bit address out on address lines, 8 bits come back)
• 8086 set up so that you could select a 64kB segment and work within it the same way you did with the 8080 (16 * 64 KB = 1 MB).

(in order in which they were introduced)

Real Mode Flat Model

• 8088, 8086, 80286
• Like a mini version of Protected Mode
• CS, DS, SS and ES set by OS and held constant to the 64K block of RAM you live in
• SP (stack pointer) starts pointing at 0xFFFF - stack size (at the top of RAM) and works its way up
• ...hence, you don't have to allocate it yourself
• WordStar and Turbo Pascal ran in this mode (64 KB for code and data!)

Real Mode Segmented Model

• Ugly
• Up to 1 MB (0x100000, 20 bits) accessible seen through a 16 bit window known as a paragraph controlled via 4 segment registers (CD, DS, SS, ES). 386 adds 2 more (FS, GS).
• these processors have 16 bit registers, so 20 bit addresssing is handled with 2 registers.
• an address can have many different representations. 0000:002D = 0001H:001D = 0002H:000D
• A 386 can put itself in real mode (vs. protected mode) to 'become' an old processor to run old DOS code
• a paragraph boundary is a memory address divisible by 16 and is a valid starting position for a segment. Therefore there are 64K segment addresses
• Segments may overlap!. They are not protected. They are a horizon beyond which a certain type of memory reference cannot go.
• Segment Registers:
  • CS = code segment. this is the segment address of the currently executing instruction. (Example: CS:IP to point to next instruction). Note, you don't change CS yourself to go to another code segment; you do a jmp, and CS gets updated for you.
  • DS = data segment. same for data/variables (Example: DS:SI)
  • SS = stack segment. same for stack data & addresses. (Example: SS:SP is where next stack op will happen)
  • ES = extra segment. a spare for specifying a memory location. (Example: ES:DI)
  • FS & GS - just 2 more ES registers.
  • They can be forced to do non-segment address stuff
• General purpose registers
  • you can only use BP, BX, SI & DI to address a memory location (e.g. mov ax, [bx] is ok; can't do mov ax, [cx])
• You have to allocate the stack yourself

Big Real Mode

from here

A small range of extended memory addresses can be accessed in real mode (Gate A20). 386 and higher x86 processors can be switched from protected mode to real mode without a reset, which allows them to operate in the Big Real Mode: a modified real mode in which the processor can access up to 4 GB of memory. BIOSs can put the processor in this mode during POST (Power On Self Test) to make access to extended memory easier.

Protected Mode Flat Model

• NT/XP & Linux
• 80386 (1986) and newer. (The 286 sort of supported it)
• CS set by OS and held constant
• apps simply cannot control this mode, an OS is required

-- MattWalsh - 15 Apr 2004