hello,everybody.I am very interested in the sourcecode of the fasm.I downloaded the sourcecode of the fasm 1.68 for dos.When i saw the file of the mode.inc, i came across a difficult problem :
I don't know the differentce between switch_real32 and switch_real16 procedures.As i can see,when the operating system operates in real mode,only the A1~A19 are effective.so what is the real32?

Search for Flat Real Mode, or UnReal mode.

This may help:
http://en.wikipedia.org/wiki/Unreal_mode

Thank you,revolution.I immediately see the link but i am still confused.since in real mode all the 4GB memory can be accessed,why there is a switch_real16? switch_real32 procedure is enough.

DOS and BIOS only work in 16-bit mode. So any interrupts (both hardware and software) must be processed in 16-bit mode, and then return back to 32-bit mode to continue assembling.

Ah,let me think for a while.As if i understand a little.
Both in the switch_real32 and switch_real16, the program will convert to protected mode and after some instructions,convert back to real mode.The switch_real32 and switch_real16 procedures function the same.So what is the difference between them?

Also,in the modes.inc ,there exists two GDTs.One is real32_GDT,the other is real16_GDT.The program is as follows :

label real32_GDTR pword
real32_GDT_limit dw 3*8-1 ; limit of GDT
real32_GDT_address dd real32_GDT ; linear address of GDT
real32_GDT rw 4 ; null descriptor
dw 0FFFFh,0,9A00h,0CFh ; 32-bit code descriptor
dw 0FFFFh,0,9200h,08Fh ; 4 GB data descriptor
label real16_GDTR pword
real16_GDT_limit dw 2*8-1 ; limit of GDT
real16_GDT_address dd real16_GDT ; linear address of GDT
real16_GDT rw 4 ; null descriptor
dw 0FFFFh,0,9A00h,0 ; 16-bit code descriptor

But the intel manual said that there is only one GDT,here are two.Why?confused.

canmeng,

CPU also has only one eax register, yet eax can contain different values. Read about lgdt instruction.

Thank you,baldr.May be what i have learned is too limited.Now i will search some articles about gdt.