Immediate Values

Question 1

Immediate Values

Answer 1

Immediate Values

• You can’t fit an arbitrary 32-bit value into a 32-bit instruction word.
• ARM data processing instructions have 12 bits of space for values in their instruction word. This is arranged as a four-bit rotate value and an eight-bit immediate value:

Question 2

Explain this picture:

Answer 2

The 4-bit rotate value stored in bits 11-8 is multiplied by two giving a range of 0-30 in steps of two.

Question 3

What can we express and not express using this scheme?

Answer 3

Using this scheme we can express immediate constants such as:

• 0x000000FF
• 0x00000FF0
• 0xFF000000
• 0xF000000F

But immediate constants such as:

• 0x000001FE
• 0xF000F000
• 0x55550000

…are not possible.

Question 4

An assembler will convert ___ values to the rotated form. Impossible values will cause an error.

Answer 4

An assembler will convert big values to the rotated form. Impossible values will cause an error.

Question 5

Some assemblers will use other tricks such as using ___ instead of MOV to form the bitwise complement of the required constant. For example the impossible instruction MOV r0,#0xFFFFFFFF could be assembled as___ r0,#0.

Answer 5

Some assemblers will use other tricks such as using MVN instead of MOV to form the bitwise complement of the required constant. For example the impossible instruction MOV r0,#0xFFFFFFFF could be assembled as MVN r0,#0.

Question 6

Remarks

The impact of this is that some constants are “ARM ______”. Some are not. Study of the numbers you’re using can sometimes reveal scope for optimisation.

Answer 6

Remarks

The impact of this is that some constants are “ARM friendly”. Some are not. Study of the numbers you’re using can sometimes reveal scope for optimisation.

Question 7

Loading Wide Values

What does this code do?

MOV r2, #0x55 ; R2 = 0x00000055 ORR r2, r2, r2, LSL #8 ; R2 = 0x00005555 ORR r2, r2, r2, LSL #16 ; R2 = 0x55555555

What does this code do?

LDR r2, =0x55555555

What does this LDR Rx,=const do?

Answer 7

Loading Wide Values

You can form constants wider than those available in a single instruction by using a sequence of instructions to build up the constant. For example:

MOV r2, #0x55 ; R2 = 0x00000055 ORR r2, r2, r2, LSL #8 ; R2 = 0x00005555 ORR r2, r2, r2, LSL #16 ; R2 = 0x55555555

…or load the value from memory:

LDR r2, =0x55555555

The pseudo-instruction LDR Rx,=const tries to form the constant in a single instruction, if possible, otherwise it will generate an LDR.