Introduction to ARM architecture variants

Some people may be very surprised that ARM can mutate. Could it be a gene mutation? Haha, in fact, to put it simply, ARM mutation means that ARM suddenly has a specific function! It is not a gene mutation! ARM is a reboot, okay?

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ARM architecture variants

First let's look at the ARM variants in a table. Understand what the ARM variants involve!

Now let me explain this picture to you:

• You don't have to worry about ARMv

• The v corresponds to the version number of the ARM instruction set

• There is an x, which means that a certain function is excluded.

It should be noted that the M variant has been a standard feature since version 4 and is no longer listed!

So this is the naming format of ARM/Thumb! Don’t worry if you don’t know what ARM architecture variants are yet, let me explain it in detail!

T variant

The full name of the t variant is the Thumb instruction set. It can be said that the T variant is a qualitative leap in ARM's efficiency. The thumb instruction set is a re-encoded subset of the ARM instruction set. It makes ARM operate more efficiently.

The ARM instruction length is 32 bits, while the Thumb instruction length is only 16 bits. For some redundant bit operations, using 16 bits is obviously more efficient than using 32 bits!

But thumb itself also has certain limitations:

• Thumb instructions generally require more instructions than ARM to complete the same operation.

• The Thumb instruction set does not include some instructions for exception handling.

Therefore, ARM cannot abandon ARM's 32-bit instructions! It can be executed more efficiently in conjunction with 16-bit Thumb instructions.

So what versions of Thumb are there so far?

1. Thumb instruction set version 1, which is applicable to ARM system version 4

2. Thumb instruction set version 2, which is applicable to ARM system version 5

M variant

We usually call it a long multiplication instruction, so pay attention:

In the previous article, we talked about long multiplication operations and long multiplication-addition operations (operations of multiplication instructions and multiplication-addition instructions). Now let me explain these two operations to you.

• Long multiplication operation: 32-bit integer multiplied by 32-bit integer to generate 64-bit integer

• Long multiply-add operation: 32-bit integer multiplied by 32-bit integer, plus 32-bit integer, to generate 64-bit integer

It was actually officially introduced in version 3, the idea was just there in version 2, and then after version 4, the M variant became a standard part of the system. We don't talk about the M variant anymore, because we use the corresponding instructions directly in the process of developing ARM!

E variant

We define it as an enhanced DSP instruction. As the application scope of ARM is getting wider and wider, in order to meet the DSP algorithm, ARM needs to enhance the processing performance of this algorithm.

So what enhancements have been made?

• Added new 16-bit data multiplication and multiply-add instructions

• Implement saturated signed number addition and subtraction instructions

• Instructions for double-word data operations, including LDRD, STDR, MCRR\MRRC

• cache prefetch instruction PLD

The E variant was first used in version 5T. It is worth noting that the early E variant did not include LDRD, STRD, MCRR\MRRC, PLD

J variant

ARM can run Android and Java thanks to this variant - Java adder Jazelle

This adder is quite amazing. Compared with the general Java virtual machine, it increases the running speed of Java code by a full 8 times, while reducing power consumption by 80%. This allows programmers to do more powerful development on it!

SIMD variants

As technology advances, people are more interested in digital media streaming. You can watch movies and play games on your phone, mostly thanks to the powerful high-performance audio and video processing technology of ARM technology, which is what we call ARM media function extension (SIMD variant)

It improves the audio and video processing performance by 4 times. ARM chips (referring to chips using ARM technology) with this function are the most popular chips on the market!

So, what are its characteristics:

• Audio\video processing performance increased by 4 times

• Provides decimal arithmetic operations

• Can perform operations on two 16-bit operands or four 8-bit operands simultaneously

• User-defined saturation operation mode

• Multiply-add/multiply-subtract operations on two sets of 16-bit operands

• 32-bit by 32-bit fractional MAC