14.2 Registers and Instructions

The register set available in real-address mode includes all the registers defined for the 8086 plus the new registers introduced by the 80386: FS, GS, debug registers, control registers, and test registers. New instructions that explicitly operate on the segment registers FS and GS are available, and the new segment-override prefixes can be used to cause instructions to utilize FS and GS for address calculations. Instructions can utilize 32-bit operands through the use of the operand size prefix.

The instruction codes that cause undefined opcode traps (interrupt 6) include instructions of the protected mode that manipulate or interrogate 80386 selectors and descriptors; namely, VERR, VERW, LAR, LSL, LTR, STR, LLDT, and SLDT. Programs executing in real-address mode are able to take advantage of the new applications-oriented instructions added to the architecture by the introduction of the 80186/80188, 80286 and 80386:

New instructions introduced by 80186/80188 and 80286.
- PUSH immediate data
- Push all and pop all (PUSHA and POPA)
- Multiply immediate data
- Shift and rotate by immediate count
- String I/O
- ENTER and LEAVE
- BOUND
New instructions introduced by 80386.
- LSS, LFS, LGS instructions
- Long-displacement conditional jumps
- Single-bit instructions
- Bit scan
- Double-shift instructions
- Byte set on condition
- Move with sign/zero extension
- Generalized multiply
- MOV to and from control registers
- MOV to and from test registers
- MOV to and from debug registers

up: Chapter 14 -- 80386 Real-Address Mode
prev: 14.1 Physical Address Formation
next: 14.3 Interrupt and Exception Handling