ARM Technical Support Knowledge Articles

What does "TDMI-S" stand for?

Applies to: ARM7TDMI-S


T : supports both ARM (32-bit) and Thumb (16-bit) instruction sets

An instruction set is a list of binary patterns, or "opcodes", which represent the different logical operations a processor can perform. Software programs can be written at different levels of abstraction, from low level "assembly code" where each written instruction typically maps onto one corresponding opcode, up to high level languages where the written program (source code) needs to be processed by a compiler which typically converts each written instruction into a whole sequence of opcodes.

ARM processors support one or more instruction sets.

The original ARM instruction set consists of 32-bit opcodes, so the binary pattern for each possible operation is four bytes long.

To improve code density, a new, smaller instruction set called "Thumb" was developed, implementing the more commonly-used parts of the ARM instruction set but encoding these in a 16-bit or 2-byte pattern (or occasionally, a pair of such opcodes).

ARM7TDMI-S supports both ARM and Thumb instruction sets, with a defined mechanism for switching between instruction sets at natural program boundaries. This instruction set architecture is called ARMv4T.

Newer ARM processors support enhanced and extended versions of one or both of these instruction sets in ARMv5, ARMv6 and ARMv7 architectures, or may support a new 32-bit instruction set for a 64-bit datapath architecture in ARMv8, either instead of or along-side ARM- and Thumb-compatible instruction sets.

D : Contains Debug extensions

The debug extensions provide the mechanism by which normal operation of the processor can be suspended for debug, including the input signal ports to trigger this behavior; for example a signal to allow a breakpoint to be indicated and a signal to allow an external debug request to be indicated.

M : Enhanced (relative to earlier ARM cores) 32x8 Multiplier block

Earlier ARM processors (prior to ARM7TDMI) used a smaller, simpler multiplier block which required more clock cycles to complete a multiplication. Introduction of this more complex 32x8 multiplier reduced the number of cycles required for a multiplication of two registers (32-bit * 32-bit) to a few cycles (data dependent). Modern ARM processors are generally capable of calculating at least a 32-bit product in a single cycle, although some of the smallest Cortex-M processors provide an implementation choice of a faster (single-cycle) or a smaller (32 cycle) 32-bit multiplier block.

I : EmbeddedICE macrocell

The EmbeddedICE macrocell consists of on-chip logic to support debug operations. In ARM7TDMI-S, this includes two instruction breakpoint and data watchpoint comparators, an Abort status register, and a debug communications channel to pass data between the target and the host. The EmbeddedICE interacts with the debug extensions, for example to signal a halt to the processor when a breakpoint is met.

-S : synthesizable (ie. distributed as RTL rather than a hardened layout)

ARM7TDMI (without the "-S" extension) was initially designed as a hard macro, meaning that the physical design at the transistor layout level was done by ARM, and licensees took this fixed physical block and placed it into their chip designs. This was the prevalent design methodology at the time. Subsequently, demand increased for a more flexible and configurable solution, so ARM moved towards delivering processor designs as a behavioral description at the "register transfer level" (RTL) written in a hardware description language (HDL), typically Verilog HDL. The process of converting this behavioral description into a physical network of logic gates is called "synthesis", and several major EDA companies sell automated synthesis tools for this purpose. A processor design distributed to licensees as an RTL description (such as ARM7TDMI-S) is therefore described as "synthesizable".

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