--- title: Instructions TARGET DECK: Obsidian::STEM FILE TAGS: x86-64 tags: - x86-64 --- ## Overview x86-64 instructions are designed so that commonly used instructions and those with fewer operands are encoded in a smaller number of bytes. Instructions range in length from 1 to 15 bytes. x86-64 assembly comes in two flavors: ATT and Intel. ATT is most common in Linux systems so I focus on that. The most important distinction between the two is operand ordering: Intel syntax lists multiple operands in reverse order compared to ATT. %%ANKI Basic x86-64 assembly comes in what two formats? Back: ATT and Intel. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Which x86-64 assembly format does Linux use? Back: ATT. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Which x86-64 assembly format does Microsoft use? Back: Intel. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the "most confusing" difference between ATT and Intel assembly? Back: Multiple operands in one are listed in reverse order relative to the other. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What term describes assembly lines with a leading `.`? Back: Directives. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Assembly directives are important for what two programs? Back: The assembler and the linker. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% ## Instruction Classes An x86-64 CPU contains a set of 16 general-purpose registers storing 64-bit values. They are used to store integers and pointers. 1 Byte | 2 Bytes | 4 Bytes | 8 Bytes | Purpose ------- | ------- | ------- | ------- | ------- `%al` | `%ax` | `%eax` | `%rax` | Return value `%bl` | `%bx` | `%ebx` | `%rbx` | Callee saved `%cl` | `%cx` | `%ecx` | `%rcx` | 4th argument `%dl` | `%dx` | `%edx` | `%rdx` | 3rd argument `%sil` | `%si` | `%esi` | `%rsi` | 2nd argument `%dil` | `%di` | `%edi` | `%rdi` | 1st argument `%bpl` | `%bp` | `%ebp` | `%rbp` | Callee saved `%spl` | `%sp` | `%esp` | `%rsp` | Stack pointer `%r8b` | `%r8w` | `%r8d` | `%r8` | 5th argument `%r9b` | `%r9w` | `%r9d` | `%r9` | 6th argument `%r10b` | `%r10w` | `%r10d` | `%r10` | Caller saved `%r11b` | `%r11w` | `%r11d` | `%r11` | Caller saved `%r12b` | `%r12w` | `%r12d` | `%r12` | Callee saved `%r13b` | `%r13w` | `%r13d` | `%r13` | Callee saved `%r14b` | `%r14w` | `%r14d` | `%r14` | Callee saved `%r15b` | `%r15w` | `%r15d` | `%r15` | Callee saved %%ANKI Basic How many general-purpose registers are available to x86-64 instructions? Back: 16 Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Cloze The x86 64-bit registers all start with prefix {`%r`}. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Cloze The x86 32-bit registers all start with prefix {`%e`}. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Instructions that generate 1-byte quantities do what to the remaining bytes of a register? Back: Leaves them alone. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Instructions that generate 2-byte quantities do what to the remaining bytes of a register? Back: Leaves them alone. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Instructions that generate 4-byte quantities do what to the remaining bytes of a register? Back: Zeroes them out. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Instructions that generate 8-byte quantities do what to the remaining bytes of a register? Back: N/A Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% There are three types of operands: * **Immediates**. These denote constant values. In ATT assembly, they are written with a `$` followed by an integer using standard C notation. * **Registers**. These denote the contents of a register. * **Memory**. These denote some memory location according to a computed address (i.e. the **effective address**). | Type | Form | Operand Value | Name | | --------- | ----------------- | ---------------------------------- | ------------------- | | Immediate | $\textdollar Imm$ | $Imm$ | Immediate | | Register | $r_a$ | $R[r_a]$ | Register | | Memory | $Imm$ | $M[Imm]$ | Absolute | | Memory | $(r_a)$ | $M[R[r_a]]$ | Indirect | | Memory | $Imm(r_b)$ | $M[Imm + R[r_b]]$ | Base + displacement | | Memory | $(r_b, r_i)$ | $M[R[r_b] + R[r_i]]$ | Indexed | | Memory | $Imm(r_b, r_i)$ | $M[Imm + R[r_b] + R[r_i]]$ | Indexed | | Memory | $(,r_i,s)$ | $M[R[r_i] \cdot s]$ | Scaled indexed | | Memory | $Imm(,r_i,s)$ | $M[Imm + R[r_i] \cdot s]$ | Scaled indexed | | Memory | $(r_b,r_i,s)$ | $M[R[r_b] + R[r_i] \cdot s]$ | Scaled indexed | | Memory | $Imm(r_b,r_i,s)$ | $M[Imm + R[r_b] + R[r_i] \cdot s]$ | Scaled indexed | %%ANKI Basic What are the three types of operands instructions can act on? Back: Immediates, registers, and memory addresses. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What are the types of source operands instructions can specify? Back: Immediates, registers, and memory addresses. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What are the types of destination operands instructions can specify? Back: Registers and memory addresses. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What does an immediate operand denote? Back: A constant value. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic In ATT syntax, how is an immediate written? Back: As a `$$` followed by an integer using standard C notation. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic In ATT syntax, how is a register written? Back: As a `%` followed by the name of the register. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $\textdollar Imm$? Back: $Imm$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $r_a$? Back: $R[r_a]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $Imm$? Back: $M[Imm]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $(r_a)$? Back: $M[R[r_a]]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $Imm(r_b)$? Back: $M[Imm + R[r_b]]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $(r_b, r_i)$? Back: $M[R[r_b] + R[r_i]]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $Imm(r_b, r_i)$? Back: $M[Imm + R[r_b] + R[r_i]]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $(,r_i,s)$? Back: $M[R[r_i] \cdot s]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $Imm(,r_i,s)$? Back: $M[Imm + R[r_i] \cdot s]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $(r_b,r_i,s)$? Back: $M[R[r_b] + R[r_i] \cdot s]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the operand value of form $Imm(r_b,r_i,s)$? Back: $M[Imm + R[r_b] + R[r_i] \cdot s]$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What distinguishes operand value $r_a$ from $(r_a)$? Back: The former denotes the register value. The latter denotes the value in memory at the address stored in $r_a$. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What values can $s$ take on in operand form $Imm(r_b,r_i,s)$? Back: $1$, $2$, $4$, or $8$. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What operand form is named "immediate"? Back: $\textdollar Imm$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What operand form is named "register"? Back: $r_a$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What operand form is named "absolute"? Back: $Imm$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What operand form is named "indirect"? Back: $(r_a)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What operand form is named "base + displacement"? Back: $Imm(r_b)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the most general operand form named "indexed" (*not* "scaled indexed")? Back: $Imm(r_b, r_i)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the most general operand form named "scaled indexed" (*not* indexed)? Back: $Imm(r_b, r_i, s)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% ### `MOV` %%ANKI Basic What four variants does `MOV` instructions take on in x86-64? Back: `movb`, `movw`, `movl`, `movq` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic How many bytes does a `movb` instruction operate on? Back: One. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic How many bytes does a `movw` instruction operate on? Back: Two. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic How many bytes does a `movl` instruction operate on? Back: Four. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic How many bytes does a `movq` instruction operate on? Back: Eight. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What combination of source and destination types is prohibited in `MOV` instructions? Back: A source and destination memory address. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after instruction `movl $0x4050,%eax`? Back: Upper 32-bits is `0` and lower 32-bits is `0x4050`. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after instruction `movq $0x4050,%rax`? Back: The 64-bit value is `0x4050`. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after instruction `movw $0x4050,%ax`? Back: The upper 48 bits are unchanged and the lower 16 bits are `0x4050`. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after instruction `movb $0x4050,%al`? Back: The upper 56 bits are unchanged and the lower 8 bits are `0x50`. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after instruction `movw $0x4050,%al`? Back: N/A. Invalid operand for instruction. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What caveat is applied to the source operand of `movq`? Back: Immediates are 32-bit two's-complement numbers sign extended to 64-bits. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What `mov` instruction is needed when working with 64-bit immediate sources? Back: `movabsq` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What purpose does `movabsq` solve that `movq` does not? Back: `movabsq` can have an arbitrary 64-bit immediate source. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movb $-1, %al ``` Back: `0x00112233445566FF` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movw $-1, %ax ``` Back: `0x001122334455FFFF` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movl $-1, %eax ``` Back: `0x00000000FFFFFFFF` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movq $-1, %rax ``` Back: `0xFFFFFFFFFFFFFFFF` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the `MOVZ` instruction class? Back: `MOV` instructions that zero extend the source to fit into the destination. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the `MOVS` instruction class? Back: `MOV` instructions that sign extend the source to fit into the destination. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What does the `movzbw` instruction do? Back: Moves a zero-extended byte to a word. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What does the `movslq` instruction do? Back: Moves a sign-extended double word to a quad word. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What does the `movslb` instruction do? Back: N/A. This instruction does not exist. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What combinatorial argument explains the number of `MOVS` instructions? Back: There exists an instruction for each smaller declaration to larger declaration. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What `MOVZ` instruction is "missing"? Back: `movzlq` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Why does there not exist a `movzlq` instruction? Back: Because `movl` already zeros out the upper bits of a destination register. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movb $0xAA, %dl movb %dl,%al ``` Back: `0x00112233445566AA` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movb $0xAA, %dl movsbq %dl,%rax ``` Back: `0xFFFFFFFFFFFFFFAA` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What is the result of `%rax` after the following instructions? ```asm movabsq $0x0011223344556677, %rax movb $0xAA, %dl movzbq %dl,%rax ``` Back: `0x00000000000000AA` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Cloze A {pointer} in C is a {memory address} in x86. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17 END%% %%ANKI Basic Dereferencing a pointer in C equates to what two operations in x86? Back: Copying the pointer into a register and then using the register in a memory reference. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17 END%% ## Bibliography * Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.