131 Week 16 - Inline Assembler

Question 1

Inline assembly

Answer 1

A way of embedding assembly code into C/C++ using the compiler.

Question 2

Reason for wanting inline assembly

Answer 2

Allows you to outsmart gcc logic.
Allows for specific hardware optimisation
Allows for close to hardware programming (drivers)
Can improve performance

Question 3

Inline assembly syntax

Answer 3

Use keyword asm or __asm__ to insert assembly. E.g., asm(“mov r0, r1”);

Question 4

Passing parameters to inline assembly

Answer 4

there are optional output operands, input operands and clobbered register list that can be included for the inline assembly.

Question 5

Inline assembly parameters syntax

Answer 5

asm(“Assembly code” : output operands : input operands : list of clobbered registers);
E.g., int a = 100, b;
asm (“movl r0, %[a];” “movl %0, r0;” : “=r” ( b ) : [a] “r” ( a ) : ”r0”);

Question 6

Input/output constraints

Answer 6

I - immediate value
J - an immediate value in the range 0-4095
M - constant in the range 0-32
m - any valid memory address
r - general register (r0-r15)
X - any operand
Adding an = before the constraint specifies it is for output.
No = before the constraint specifies it is for input

Question 7

Using inputs/outputs

Answer 7

Input/output for the inline assembly for inline assembly is defined by:
[assemblyName] “constraint” (codeName)
You can access the input/output using %assemblyName
E.g., [B] “=r” (b) for using a C variable b as an input register which can be used in the inline assembly through %B

Question 8

Clobbered registers

Answer 8

Clobbered registers are registers that are modified during the inline assembly but not stated as an input or output register.
They need to be declared as otherwise the compiler does not know the register is being used and could overwrite it or optimise incorrectly.
Status registers may also need to be clobbered such as “cc” (conditional code flags) and “memory” (flags if instruction accesses unknown memory).

Question 9

Register allocation

Answer 9

A compiler must assign variables to a processor register but 2 variables cannot be assigned to the same register at any point.
Can use spilling to store variable values if needed.

Question 10

Volatile

Answer 10

Some assembly code must be executed exactly where it is put in the code and exactly how it is written e.g., when working with hardware or memory-mapped IO.
The volatile keyword tells the compiler not to move the code or not to remove it as it may do these while trying to optimise code.
Syntax:
volatile asm(“assembly”);
__volatile__ __asm__(“assembly”);