The general differences between the Thumb instruction set and the ARM instruction set are dealt with under the following headings:
- Branch instructions
- Data processing instructions
- Single register load and store instructions
- Multiple register load and store instructions.
There are no Thumb coprocessor instructions, no Thumb semaphore instructions, and no Thumb instructions to access the CPSR or SPSR.
Branch instructions
These instructions are used to:
- branch backwards to form loops
- branch forward in conditional structures
- branch to subroutines
- change the processor from Thumb state to ARM state.
Program-relative branches, particularly conditional branches, are more limited in range than in ARM code, and branches to subroutines can only be unconditional.
Data processing instructions
These operate on the general-purpose registers. In many cases, the result of the operation must be put in one of the operand registers, not in a third register. There are fewer data processing operations available than in ARM state. They have limited access to registers r8 to r15.
The ALU status flags in the CPSR are always updated by these instructions except when MOV or ADD instructions access registers r8 to r15. Thumb data processing instructions that access registers r8 to r15 cannot update the flags.
Single register load and store instructions
These instructions load or store the value of a single low register from or to memory. In Thumb state they can only access registers r0 to r7.
Multiple register load and store instructions
LDM and STM load from memory and store to memory any subset of the registers in the range r0 to r7.
PUSH and POP instructions implement a full descending stack using the stack pointer (r13) as the base. In addition to transferring r0 to r7, PUSH can store the link register and POP can load the program counter.
中文：
Thumb 指令集没有协处理器指令,信号量指令以及访问 CPSR 或 SPSR 的指令,没有乘加指令及 64 位乘法指令等,且指令的第二操作数受到限制;除了跳转指令 B 有条件执行功能外,其它指令均为无条件执行;大多数 Thumb 数据处理指令采用 2 地址格式.Thumb指令集与 ARM 指令的区别一般有如下几点:
跳转指令
程序相对转移,特别是条件跳转与 ARM 代码下的跳转相比,在范围上有更多的限制,转向子程序是无条件的转移.
数据处理指令
数据处理指令是对通用寄存器进行操作,在大多数情况下,操作的结果须放入其中一个操作数寄存器中,而不是第 3 个寄存器中.数据处理操作比 ARM 状态的更少,访问寄存器 R8~R15 受到一定限制.除 MOV 和 ADD 指令访问器 R8~R15 外,其它数据处理指令总是更新 CPSR 中的 ALU 状态标志.访问寄存器 R8~R15 的 Thumb 数据处理指令不能更新 CPSR 中的 ALU 状态标志.
单寄存器加载和存储指令
在 Thumb 状态下,单寄存器加载和存储指令只能访问寄存器 R0~R7
批量寄存器加载和存储指令
LDM 和 STM 指令可以将任何范围为 R0~R7 的寄存器子集加载或存储. PUSH 和 POP 指令使用堆栈指令 R13 作为基址实现满递减堆栈.除 R0~R7 外,PUSH 指令还可以存储链接寄存器 R14,并且 POP 指令可以加载程序指令PC
