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我找到了组装说明

cltd

通过反汇编英特尔架构上的代码。我发现的描述是，它清除了 %edx 寄存器，但我不明白会发生什么......谁能解释一下该命令的确切作用？

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5 回答 5

28

cltd是cdq( reference ) 的别名，其符号扩展eax为edx:eax.

这在实践中意味着它被（符号位）edx的最高有效位填充。eax例如， if eaxis0x7F000000 edx会变成0x00000000after cdq。如果eax是0x80000000 edx会变成0xFFFFFFFF.

于 2013-06-18T13:41:15.157 回答
4

cltd将有符号长转换为有符号双长

如果您想查看所发生情况的图表，请跳转到http://download.intel.com/products/processor/manual/325462.pdf的第 160 页（第 681 页上的更多详细信息）

于 2013-06-18T13:41:05.347 回答
1

比这更简单，例如：

如果该位小于0x7F000000，即小于十进制的 127，则其“正数”介于 0 到 127 之间。

如果该位大于0x80000000十进制的 128，则其“负数”介于 -1 和 -128 之间。

所以 edx 会得到0x00000000pos 和0xffffffffneg。

例如，当您知道 eax 是 pos 号而不是xor rex, edx.

PScltd可能无法像0x99在某些 asm 编译器中那样编译。更好用cdq

于 2018-05-13T10:36:30.100 回答
1

Michael 提到的文档摘要：

Intel   AT&T   From   To
CBW     CBTW   AL     AX
CWDE    CWTL   AX     EAX
CWD     CWTD   AX     DX:AX
CDQ     CLTD   EAX    EDX:EAX
CDQE    CLTQ   EAX    RAX
CQO     CQTO   RAX    RDX:RAX

现在一些注释代码片段：

/* Quad to Octo: top bit is zero: extend with zeroes. */
mov $0x7FFFFFFFFFFFFFFF, %rax
mov $0x123456789ABCDEF0, %rdx
cqto
mov %rax, %r12
mov %rdx, %r13
/* rax is unchanged. */
LKMC_ASSERT_EQ(%r12, $0x7FFFFFFFFFFFFFFF)
/* rdx is filled with zeros. */
LKMC_ASSERT_EQ(%r13, $0)

/* Quad to Octo: top bit is one: extend with ones. */
mov $0x8000000000000000, %rax
mov $0x123456789ABCDEF0, %rdx
cqto
mov %rax, %r12
mov %rdx, %r13
LKMC_ASSERT_EQ(%r12, $0x8000000000000000)
LKMC_ASSERT_EQ(%r13, $0xFFFFFFFFFFFFFFFF)

/* Intel equivalent syntax also accepte by GNU GAS. */
mov $0x7FFFFFFFFFFFFFFF, %rax
mov $0x123456789ABCDEF0, %rdx
cqo
mov %rax, %r12
mov %rdx, %r13
LKMC_ASSERT_EQ(%r12, $0x7FFFFFFFFFFFFFFF)
LKMC_ASSERT_EQ(%r13, $0)

/* Smaller size example: Double to Quad.
 * Also zeroes top 32-bits of RDX like many 32 to 64 operaions. */
mov $0xFFFFFFFF7FFFFFFF, %rax
mov $0x123456789ABCDEF0, %rdx
cltd
mov %rax, %r12
mov %rdx, %r13
LKMC_ASSERT_EQ(%r12, $0xFFFFFFFF7FFFFFFF)
LKMC_ASSERT_EQ(%r13, $0)

/* Even smaller size example: Word to Doubleword.
 * Unlike the 32-bit one, does not zero out the top 32-bits of RDX. */
mov $0xFFFFFFFFFFFF7FFF, %rax
mov $0x123456789ABCDEF0, %rdx
cwtd
mov %rax, %r12
mov %rdx, %r13
LKMC_ASSERT_EQ(%r12, $0xFFFFFFFFFFFF7FFF)
LKMC_ASSERT_EQ(%r13, $0x123456789ABC0000)

和

/* CLTQ: top bit is zero: extend with zeroes. */
mov $0x123456787FFFFFFF, %rax
cltq
LKMC_ASSERT_EQ(%rax, $0x000000007FFFFFFF)

/* CLTQ: top bit is one: extend with ones. */
mov $0x1234567880000000, %rax
cltq
LKMC_ASSERT_EQ(%rax, $0xFFFFFFFF80000000)

/* CWTL: zeroes top 32-bits. */
mov $0x123456789ABC8EF0, %rax
cwtl
LKMC_ASSERT_EQ(%rax, $0xFFFF8EF0)
CWTL

/* CBTW. */
mov $0x123456789ABCDE80, %rax
cbtw
LKMC_ASSERT_EQ(%rax, $0x123456789ABCFF80)
CWTL

可运行的 GitHub 上游：

于 2019-06-21T08:47:23.447 回答