X86lite Specification

Overview

X86lite is an abstract, 64-bit signed integer-only subset of the Intel X86-64 machine architecture. The X86lite instruction set is tiny by comparison to full X86, yet it still provides a sufficient compilation target for the CIS341 course compiler projects.

This document explains the X86lite machine model and its instruction set, and is intended for use as a reference manual. Information about the full X86 architecture can be found on the Intel web pages. The CIS341 course infrastructure provides OCaml interfaces for manipulating X86lite assembly programs and tools for creating X86-64 executables using the system assembler and linker.

X86lite Machine State

The X86lite machine state consists of sixteen general-purpose 64-bit registers, an instruction pointer register that can only be manipulated indirectly by control flow instructions, three condition flags, and a memory consisting of 2⁶⁴ bytes. Instructions are represented as 8-byte words using an unspecified fixed-length encoding.

Register file

The 16 64-bit registers in X86lite and their common uses in the full X86 architecture are given below. In X86lite, most of the registers can be used for general purpose calculation, but some X86lite instructions make special use of some of the registers; see the instruction descriptions below.

Register	Description / common use on X86
rax	General purpose accumulator
rbx	Base register, pointer to data
rcx	Counter register for strings and loops
rdx	Data register for I/O
rsi	Pointer register, string source register
rdi	Pointer register, string destination register
rbp	Base pointer, points to the stack frame
rsp	Stack pointer, points to the value at the top of the stack
r08–r15	General purpose

In addition, the instruction pointer register rip contains the address of the next instruction to execute. The address in rip is used to load the next instruction to execute, then rip is increased by the size of the instruction (always 8-bytes, since we use a fixed-length encoding), and then the instruction is executed.

Condition flags

The X86 architecture provides conditional branch and conditional move instructions. The processor maintains a set of bit-sized flags to keep track of conditions arising from arithmetic and comparison operations. These condition flags are tested by the conditional jump and move instructions; the flags are set by the arithmetic instructions. X86lite provides only three condition flags (the full X86 architecture has several more).

Condition flag	Description
OF	Overflow set when the result is too big or too small to fit in a 64-bit value and cleared otherwise. This is overflow/underflow for signed (two’s complement) 64-bit arithmetic.
SF	Sign equal to the most significant bit of the result (0 = positive, 1 = negative)
ZF	Zero set if the result is 0 and cleared otherwise

Memory, addresses, and the stack

The X86lite memory consists of 2⁶⁴ bytes numbered 0x0000000000000000 through 0xffffffffffffffff. All of the X86lite instructions operate on 8-byte quadwords, but memory is byte-addressable. That is, unaligned memory accesses are legal.

The only general-purpose register that is treated specially by the X86lite ISA is rsp, which contains the address of the top of the stack of the executing program. By convention on X86 machines, the program stack starts at the high addresses of virtual memory and grows toward the low addresses. Instructions like pushq, popq, callq, and retq, increment and decrement rsp as needed to maintain this invariant.

X86lite Operands and Condition Codes

This section describes the X86lite instruction set.

Operands

X86lite instructions manipulate data stored in memory or in registers. The values operated on by a given instruction are described by operands, which are constant values like integers and statically known memory addresses, or dynamic values such as the contents of a register or a computed memory address.

Operands can take one of several forms, described below:

Operand kind	Description
Imm : imm	An immediate, constant literal of size 64-bits or a symbolic label that is resolved by the assembler/linker/loader to a 64-bit constant. Label values typically denote targets of Jmp or Call instructions.
Reg : reg	One of the sixteen machine registers, or rip. The value of a register is its contents.
Ind1 : imm	An indirect address consisting only of a displacement by a literal or symbolic label immediate value. An example use is leaq _Label, %rax, which loads the address denoted by the symbolic label into register rax.
Ind2 : reg	An indirect reference to an address held in a register. For example, movq %rbx, (%rax) moves the contents of register rbx into the memory location at the address held in rax.
Ind3 : imm * reg	An indirect reference to an offset of an address held in a register. For example, movq %rbx, 8(%rax) moves the contents of register rbx into the memory location 8 bytes past the address held in rax.

In their full generality, and X86 indirect reference operand consists of three optional components:

[base : reg] [index : reg, scale : int64] [disp : (int64 | Label)]

The effective address denoted by an indirect address is calculated by:

addr(Ind) = base + (index * scale) + disp

In the formula above, a missing optional component’s value is 0. For the purposes of X86lite, we disregard the index and scale parts, which yields the three combinations given by Ind1, Ind2, and Ind3 described above.

When an Ind operand is used as a value (not a location) the operand denotes Mem[addr(Ind)], the contents of the machine memory at the effective address denoted by Ind.

Condition codes

The X86lite cmpq SRC1, SRC2 instruction is used to compare two 64-bit operands (SRC1 and SRC2). It works by subtracting SRC1 from SRC2 (i.e., SRC2 - SRC1), setting the condition flags according to the result (the actual result of the subtraction is ignored).

The X86lite conditional branch (J) and conditional set (setb) instructions specify condition codes that look at the condition flags to determine whether or not the condition is satisfied. The eight condition codes and their interpretation in terms of condition flags are given in the following table:

Condition code	Description
eq	Equals: This condition holds when ZF is set. (Intuitively SRC1 = SRC2 when SRC2 - SRC1 = 0.)
neq	Not equals: This condition holds when ZF is not set.
lt	(Signed) less than: This condition holds when SF does not equal OF. Equivalently, this condition holds when (SF = 1 and OF = 0) or (SF = 0 and OF = 1). The first case holds when the result of SRC2 - SRC1 is negative and there has been no overflow, the second case holds when the result of SRC2 - SRC1 is positive and there has been an overflow.
le	(Signed) less than or equal: This condition holds when (SF is not equal to OF) or ZF is set. This is equivalent to (lt or eq).
gt	(Signed) greater than: This condition holds when (not le) holds.
ge	(Signed) greater than or equal: This condition holds when (not lt) holds. Or, equivalently, if SF equals OF.

X86lite Instructions

There are only about 20 instructions in the X86lite architecture. Together, they provide basic signed arithmetic over 64-bit integers, logical operations, data movement between registers and memory, and control-flow operations for branches and jumps. In general, instructions that involve two operands must not use two memory (Ind) operands.

When an operand appears on the right-hand side of the ← symbol in the instruction descriptions below, it is interpreted as a value, computed as described above. When an operand appears on the left-hand side of an ← symbol, it is interpreted as a location. The location of a register operand is the register itself; the location of an Ind operand is the memory location at address addr(Ind). Immediate values and labels do not denote valid locations.

In the following table, the Flags column indicates which condition flags are affected by the operation. The symbol --- means that no condition flags are set (i.e., the operation does not modify the condition flags). The presence of the symbols SF, ZF, and OF indicate that these flags are set as described in the condition flags section. A * next to a flag indicates special handling. Note that overflow conditions for all arithmetic operations are defined per instruction.

Arithmetic Instructions

Arithmetic Instructions

Instruction	Operation	Flags	Comments
negq DEST	DEST ← -DEST	SF, ZF, OF	Two’s complement negation. Note that flag OF is set to 1 if DEST is MIN_INT, and 0 otherwise.
addq SRC, DEST	DEST ← DEST +64 SRC	SF, ZF, OF	Signed integer addition. Let D64 be the 64-bit sign extension of DEST and S64 be the 64-bit sign extension of SRC. The result R64 = DEST +64 SRC is the 64-bit truncation of R64, which is obtained by 64-bit addition R64 = D64 +64 S64.
subq SRC, DEST	DEST ← DEST -64 SRC	SF, ZF, OF	Signed integer subtraction. This operation can be computed using arithmetic negation and addition. Let D64 be the 64-bit sign extension of DEST and S64 be the 64-bit sign extension of SRC. The result R64 = DEST -64 SRC is the 64-bit truncation of R64, which is obtained by the 64-bit computation R64 = D64 -64 S64.
imulq SRC, Reg	Reg ← Reg *64 SRC	SF, ZF, OF	Signed integer multiply. Let D64 be the 64-bit sign extension of Reg and S64 be the 64-bit sign extension of SRC. The result R64 = Reg *64 SRC is the 64-bit truncation of R64, which is obtained by 64-bit multiplication R64 = D64 *64 S64. Flag OF is set when R64 cannot be represented as a 64-bit sign-extended integer. Flags ZF and SF are undefined.
incq DEST	DEST ← DEST +64 1	SF, ZF, OF	Computes the 64-bit successor of DEST. Flags are set as in addq.
decq DEST	DEST ← DEST -64 1	SF, ZF, OF	Computes the 64-bit predecessor of DEST. Flags are set as in subq.

Logic Instructions

Logic Instructions

Instruction	Operation	Flags	Comments
notq DEST	DEST ← not DEST	---	One’s complement (logical) negation.
andq SRC, DEST	DEST ← DEST AND SRC	SF, ZF, OF	Logical AND. Flag OF is always set to 0.
orq SRC, DEST	DEST ← DEST OR SRC	SF, ZF, OF	Logical OR. Flag OF is always set to 0.
xorq SRC, DEST	DEST ← DEST XOR SRC	SF, ZF, OF	Logical XOR. Flag OF is always set to 0.

Bit-manipulation Instructions

Instruction	Operation	Flags	Comments
sarq AMT, DEST	DEST ← DEST >> AMT	SF*, ZF*, OF*	Arithmetic shift DEST right by AMT, replicating the sign bit for the vacated spaces. AMT must be a Imm or rcx operand. If AMT = 0 flags are unaffected. Otherwise the flags SF and ZF are set as usual. The OF flag is set to 0 if the shift amount is 1 (and is otherwise unaffected).
shlq AMT, DEST	DEST ← DEST << AMT	SF*, ZF*, OF*	Bitwise shift DEST left by AMT. AMT must be a Imm or rcx operand. If AMT = 0 flags are unaffected. Otherwise, flags SF and ZF are set as usual. If the shift amount is 1, then OF is set to 1 if the top two bits (i.e., the two most-significant bits) of the original value of DEST are different and to 0 if the top two bits are the same. If the shift amount is not 1, then OF should not be modified.
shrq AMT, DEST	DEST ← DEST >>> AMT	SF*, ZF*, OF*	Bitwise shift DEST right by AMT inserting 0’s for the vacated spaces. AMT must be a Imm or rcx operand. If AMT = 0 flags are unaffected, otherwise the SF is set to the most significant bit of the result, and the ZF flag is set if and only if the result is zero. OF is set to the most-significant bit of the original operand if the shift amount is 1 (and is otherwise unaffected).
setb CC, DEST	DEST’s lower byte ← if CC then 1 else 0	---	If condition code CC holds in the current state, move 1 into the lower byte of DEST; otherwise move 0 into the lower byte.

Data-movement Instructions

Instruction	Operation	Flags	Comments
leaq Ind, DEST	DEST ← addr(Ind)	---	Load effective address of Ind, which must be an operand of type ind (see operands). This instruction calculates a pointer into memory and stores it in DEST.
movq SRC, DEST	DEST ← SRC	---	Copy the value of SRC to the location denoted by DEST
pushq SRC	rsp ← rsp - 8; Mem[rsp] ← SRC	---	Push a 64-bit value onto the stack: decrement rsp by 8 to allocate the new stack slot and then store SRC in the resulting memory address.
popq DEST	DEST ← Mem[rsp]; rsp ← rsp + 8	---	Pop the top of the stack into DEST: Load the value pointed to by rsp from memory and then increment rsp by 8.

Control-flow and condition Instructions

Instruction	Operation	Flags	Comments
cmpq SRC1 SRC2	← SRC2 -64 SRC1	SF*, ZF*, OF*	Compare SRC1 to SRC2 by setting all condition flags as though the instruction subq SRC1 SRC2 was executed. Does not change register or memory contents.
jmp SRC	rip ←; SRC	---	Jump to the machine instruction at the address given by the value of SRC. Sets the program counter.
callq SRC	pushq %rip rip ← SRC	---	Call a procedure: Push the program counter (which, at this point, contains the address of the instruction <strong>after</strong> the callq instruction) to the stack (decrementing rsp) and then jump to the machine instruction at the address given by the value of SRC.
retq	popq %rip	---	Return from a procedure: Pop the current top of the stack into rip (incrementing rsp); this instruction effectively jumps to the address at the top of the stack.
j CC SRC	rip ← if CC then SRC else *	---	Conditional jump: If the condition code CC holds in the current state, set rip to SRC; otherwise set rip to the next instruction (i.e. fallthrough).