ARM、Thumb 和 Thumb 2 指令编码之间有什么区别? [英] What is the difference between the ARM, Thumb and Thumb 2 instruction encodings?

问题描述

我对指令集有点困惑.有 Thumb、ARM 和 Thumb 2.从我读到的 Thumb 指令都是 16 位的，但在 ARMv7M 用户手册(第 vi 页)中有 Thumb 16 位和 Thumb 32 位指令提及.

I am a bit confused about instruction sets. There are Thumb, ARM and Thumb 2. From what I have read Thumb instructions are all 16-bit but inside the ARMv7M user manual (page vi) there are Thumb 16-bit and Thumb 32-bit instructions mentioned.

现在我必须克服这种困惑.据说Thumb 2支持16位和32位指令.那么 ARMv7M 实际上是否支持 Thumb 2 指令而不仅仅是 Thumb?

Now I have to overcome this confusion. It is said that Thumb 2 supports 16-bit and 32-bit instructions. So is ARMv7M in fact supporting Thumb 2 instructions and not just Thumb?

还有一件事.我可以说 Thumb(32 位)与同样是 32 位的 ARM 指令相同吗?

One more thing. Can I say that Thumb (32-bit) is the same as ARM instructions which are allso 32-bit?

推荐答案

哦，ARM 和他们愚蠢的命名...

Oh, ARM and their silly naming...

这是一个常见的误解，但官方没有Thumb-2 指令集"这样的东西.

It's a common misconception, but officially there's no such thing as a "Thumb-2 instruction set".

忽略 ARMv8(所有内容都被重命名，而 AArch64 使事情复杂化)，从 ARMv4T 到 ARMv7-A 有两个指令集:ARM 和 Thumb.它们都是32 位"，因为它们在具有 32 位地址的 32 位宽寄存器中对高达 32 位宽的数据进行操作.事实上，在它们重叠的地方，它们代表完全相同的指令——只是指令编码不同，而 CPU 实际上只有两个不同的解码前端到它的管道，它可以在它们之间切换.为清楚起见，我现在将故意避免使用术语32 位"和16 位"...

Ignoring ARMv8 (where everything is renamed and AArch64 complicates things), from ARMv4T to ARMv7-A there are two instruction sets: ARM and Thumb. They are both "32-bit" in the sense that they operate on up-to-32-bit-wide data in 32-bit-wide registers with 32-bit addresses. In fact, where they overlap they represent the exact same instructions - it is only the instruction encoding which differs, and the CPU effectively just has two different decode front-ends to its pipeline which it can switch between. For clarity, I shall now deliberately avoid the terms "32-bit" and "16-bit"...

ARM 指令具有固定宽度的 4 字节编码，需要 4 字节对齐.Thumb 指令具有需要 2 字节对齐的可变长度(2 或 4 字节，现在称为窄"和宽")编码 - 大多数指令具有 2 字节编码，但 bl 和 blx 一直有 4 字节编码^*.真正令人困惑的是 ARMv6T2，它引入了Thumb-2 技术".Thumb-2 不仅包括向 Thumb 添加加载更多指令(主要使用 4 字节编码)使其几乎与 ARM 相同，而且还扩展了执行状态以允许有条件地执行大多数 Thumb 指令，最后引入了一个全新的汇编语法(UAL，统一汇编语言")取代了之前单独的 ARM 和 Thumb 语法，并允许编写一次代码，无需修改即可将其组装到任一指令集.

ARM instructions have fixed-width 4-byte encodings which require 4-byte alignment. Thumb instructions have variable-length (2 or 4-byte, now known as "narrow" and "wide") encodings requiring 2-byte alignment - most instructions have 2-byte encodings, but bl and blx have always had 4-byte encodings^*. The really confusing bit came in ARMv6T2, which introduced "Thumb-2 Technology". Thumb-2 encompassed not just adding a load more instructions to Thumb (mostly with 4-byte encodings) to bring it almost to parity with ARM, but also extending the execution state to allow for conditional execution of most Thumb instructions, and finally introducing a whole new assembly syntax (UAL, "Unified Assembly Language") which replaced the previous separate ARM and Thumb syntaxes and allowed writing code once and assembling it to either instruction set without modification.

Cortex-M 架构仅实现 Thumb 指令集 - ARMv7-M (Cortex-M3/M4/M7) 支持大部分Thumb-2 技术"，包括条件执行和 VFP 指令编码，而 ARMv6-M(Cortex-M0/M0+) 仅以少量 4 字节系统指令的形式使用 Thumb-2.

The Cortex-M architectures only implement the Thumb instruction set - ARMv7-M (Cortex-M3/M4/M7) supports most of "Thumb-2 Technology", including conditional execution and encodings for VFP instructions, whereas ARMv6-M (Cortex-M0/M0+) only uses Thumb-2 in the form of a handful of 4-byte system instructions.

因此，新的 4 字节编码(以及后来在 ARMv7 修订版中添加的那些)仍然是Thumb 指令 - 它们的Thumb-2"方面是它们可以 4 字节编码，并且它们(大部分)可以通过 it 有条件地执行(而且，我想，它们的助记符仅在 UAL 中定义).

Thus, the new 4-byte encodings (and those added later in ARMv7 revisions) are still Thumb instructions - the "Thumb-2" aspect of them is that they can have 4-byte encodings, and that they can (mostly) be conditionally executed via it (and, I suppose, that their menmonics are only defined in UAL).

_{* 在 ARMv6T2 之前，关于 bl(或 blx)是作为 4 字节指令执行还是作为一个一对 2 字节指令.体系结构定义是后者，但由于它们只能按顺序成对执行，因此出于性能原因将它们融合为一条指令不会有什么损失(除了中途中断的能力).ARMv6T2 只是在融合的单指令执行方面重新定义了事物}

_{* Before ARMv6T2, it was actually a complicated implementation detail as to whether bl (or blx) was executed as a 4-byte instruction or as a pair of 2-byte instructions. The architectural definition was the latter, but since they could only ever be executed as a pair in sequence there was little to lose (other than the ability to take an interrupt halfway through) by fusing them into a single instruction for performance reasons. ARMv6T2 just redefined things in terms of the fused single-instruction execution}

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