The ARM processor from the point of view of an assembler programmer

The most important difference between assembler programming and high-level language programming
- Important fact about high-level language programming:
- Important fact about assembler language programming:

The registers in a "general" processor (i.e.: in general)

Recall the structure of the CPU (= Central Processing Unit, or processor) :
When you write assembler programs, you will need to use/access the registers within the CPU (processor):

Each register has a unique name

Unfortunately, each type of CPU (processor) has different names for its registers

Example:

CPU Name of some of the registers ------------------- ------------------------------- Intel processor rax, rbx, rcx, rdx, ... ARM processor r0, r1, r2, r3, ...

When you write an assembler program, you will need to use the registers in the CPU inside your assembler program

Because the registers' names are different for different processor type, the assembler programs written for one type of processor cannot be compiled and run on another type of processor

Fortunately:

Assembler programming techiques are similar regardless of CPU type
So the assembler programming techiques that you will learn in CS255 can be apply to any assembler language
When you need to write assembler programs for another processor, all you need to find out are:

So the first step in learning assembler programming in the ARM assembler language, you will need to know the CPU structure of the ARM processor (CPU)

What an assembler programmer need to know about the register in order to write assembler programs
- The assembler programmer must know these facts about the registers:
  We will learn assembler programming using the ARM processor.
  So I will describe these properties of the ARM registers now.
CPU structure of the ARM processor
- The ARM processor (CPU) has the following registers:
  I will explain the use of the register below.
- Size of ARM registers:
  - Each register in the ARM processor consists of 4 bytes:

The different registers in the ARM processor

There are 2 kinds of registers in a CPU:

General purpose registers
Special purpose registers

The ARM processor has 12 general purpose registers with the following register names:

R0 (or r0 the register names are not case senstive)
R1
R2
...
R10
R11 -- the register R11 is designed as the frame pointer (FP) register which will be explained (very) later in this course

Although register R11 is a "general purpose" register, in practice, we have assign it for a specific purpose....

So register R11 may be considered as a special purpose register (i.e.: don't use register r11 to store value for computation !)

The ARM processor has the following special purpose registers with the following register names:

R12 or the Intra Procedure (IP) Call register

R13 or the Stack Pointer (SP) register

The stack pointer (SP) register contains the address of the top of the system stack
We will learn to use the SP register when we discuss how to write function calls in assembler programming

R14 or the Link Register (LR)

The Link Register (SP) contains the return address of the calling program
We will learn to use the LR register when we discuss how to write function calls in assembler programming

R15 or the Program Counter (PC)

The Program Counter (PC) contains the address of the next program instruction
We have explained the use of the Program Counter in this webpage (lecture): click here)

CPSR (= Current Program Status Register)

The CPSR register contains indications ("flags") of the status of the current instruction execution

For the assembler programmer, the following flags (indications) are especially important:

N flag: this flag (bit) is set (= 1) if the result of the computation is a negative value
Otherwise, this flag (bit) is reset (= 0)
Z flag: this flag (bit) is set (= 1) if the result of the computation is equal to 0 (zero)
Otherwise, this flag (bit) is reset (= 0)
V flag: this flag (bit) is set (= 1) if the result of the computation has caused overflow (later)
Otherwise, this flag (bit) is reset (= 0)
C flag: this flag (bit) is set (= 1) if the result of the computation produced a carry
Otherwise, this flag (bit) is reset (= 0)

As an assembler programmer, you do not need to keep track of the status of these 4 flags.

These 4 flags are used when we compare 2 values

To compare the values x and y, we compute the subtraction x − y.

The setting of these 4 flags will tell us if:

x > y x >= y x < y x <= y x == y or x != y

This topic will be discussed later when we discuss how if-statements in Java is executed in assembler language

Recommended (or commonly used) register names for assembler programming

Although you can use the registers names R0, R1, R2, ...., R15 to identify the registers in the ARM processor, the UNIX assembler (= compiler) also recognize the following commonly used register names:

ARM register Commonly used register name in assembler programming --------------- ------------------------------------------------------ R0 r0 or R0 R1 r1 or R1 R2 r2 or R2 R3 r3 or R3 R4 r4 or R4 R5 r5 or R5 R6 r6 or R6 R7 r7 or R7 R8 r8 or R8 R9 r9 or R9 R10 r10 or R10 R11 fp (frame pointer) R12 ip (intra procedure) R13 sp (stack pointer) R14 lr (link register) R15 pc (program counter)

Assembler programs will not use the CPSR register directly and will therefore not refer to the CPSR register by a register name.