Assembler
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To describe:
- Flags
- Segments
- CPL/DPL
- IDT/GDT(/LDT)
Contents
Segments
Segment registers: cs,es,ds,ss,fs,gs
Bits 0,1 describe the RPL , request privilege level
Bit 2 describes if the LDT is used or not
Bits 3 to 15 contain the offset into the GDT or LDT table (when shifted left by 3)
example:
CS of 8 = 1000b = 1 0 00 : RPL=0, LDT=0, so GDT is used, offset in GDT table is (1 << 3) = 8
CS of 0x23 = 100011b = 100 0 11 : RPL=3, LDT=0 (GDT), offset in GDT table is 100b=4, (4 << 3) = 32
Note that even though 64-bit mode is used, bits 3 to 15 still only need to be shifted by 3 to point to the proper offset
GDT
The gdt is a table of descriptors that describe what should happen when entering a specific segment and setting it's rights. (What access rights, the limits, if it's data or code, etc...)
IDT
The IDT is a table of descriptors that describe what should happen when an interrupt occurs. It contains the used code segment, and the EIP/RIP address to call, but also information like the DPL of the interrupt and if it's a callgate, taskgate or interrupt gate
Flags
ID, VIP, VIF, AC, VM, RF, NT, IOPL, OF, DF, IF, TF, SF, ZF, AF, PF, CF
Opcodes
Most commonly used opcodes:
.... ADD
SUB
MOV
NOP = No Operation , usually used when removing the code that decreases life
XOR
....
