Code Layout
.data
@variables
.text
.global main
@code
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Data Types
Type |
Mnemonic |
Bytes size |
Word |
|
4 |
Half word |
h |
2 |
Byte |
b |
1 |
Registers
# |
Purpose |
Description |
R0 - R12 |
General Purpose |
Stores temporary values, pointers... |
R13 |
SP - Stack Pointer |
Top of the stack. Allocate space to the stack by substracting the value in bytes that we want to allocate from the stack pointer. |
R14 |
LR - Link Register |
When a function call is made, LR gets pdated with a memory address referencing the next instruction where the function was initiated from |
R15 |
PC - Program Counter |
Automatically incremented by the size of the instruction executed |
CPSR - Current Program Status Register
Flag |
Meaning |
Enable if result of the instruction yields a... |
N |
Negative |
Negative number |
Z |
Zero |
Zero value |
C |
Carry |
Value that requires a 33rd bit to be fully represented |
V |
Overflow |
Value that cannot be represented in 32 bit two's complement |
Flexible operand
#123 |
Inmediate value |
Rx |
Register x |
Rx, LSL n |
Register x with logical shift left by n bits |
Rx, LSR n |
Register x with logical shift right by n bits |
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Syntax
MNEMONIC{S}{condition} {Rd}, Operand1, Operand2 |
Mnemonics
MNEMONIC |
Description |
{S} |
An optional suffix. If S is specified, the condition flags are updated on the result of the operation |
{condition} |
Condition that is needed to be met in order for the instruction to be executed |
{Rd} |
Register destination for storing the result of the instruction |
Operand1 |
First operand. Either a register or an inmediate value |
Operand2 |
Second (flexible) operand. Either an inmediate value (number) or a register with an optional shift |
Common Instructions
Instruction |
Description |
MOV |
Move data |
MVN |
Move and negate |
ADD |
Addition |
SUB |
Substraction |
MUL |
Multiplication |
LSL |
Logical Shift Left |
LSR |
Logical Shift Right |
ASR |
Arithmetic Shift Right |
CMP |
Compare |
CMN |
Compare and negate |
AND |
Bitwise AND |
ORR |
Bitwise OR |
EOR |
Bitwise XOR |
LDR |
Load |
STR |
Store |
LDM |
Load Multiple |
STM |
Store Multiple |
B |
Branch |
BL |
Branch with Link |
BX |
Branch and eXchange |
BLX |
Branch with Link and eXchange |
BIC |
Bit Clear |
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Address modes
|
Offset str r2, [r1, #2]
Store the value found in R2 to the memory address found in R1 plus 2. Base register unmodified. |
|
Pre-indexed str r2, [r1, #4]!
Store the value found in R2 to the memory address found in R1 plus 4. Base register (R1) modified: R1= R1+4 |
|
Post-indexed ldr r3, [r1], r2, LSL#2
Load the value at memory address found in R1 to the register R3. Then modify base register: R1 = R1+R2<<2 |
Syntax:
STR Ra, [Rb, imm]
LDR Ra, [Rc, imm]
If there is a !, its prefix address mode ldr r3, [r1, #4]!
ldr r3, [r1, r2]!
ldr r3, [r1, r2, LSL#2]!
If the base register is in brackets by itself, it’s postfix address mode
ldr r3, [r1], #4
ldr r3, [r1], r2
ldr r3, [r1], r2, LSL#2
Anything else, offset address mode:
ldr r3, [r1, #4]
ldr r3, [r1, r2]
ldr r3, [r1, r2, LSL#2]
Conditionals
Mnemonic |
Description |
Flags |
EQ |
Equals |
Z=1 |
NE |
Non equals |
Z=0 |
HI |
Higher than (NS) |
C=1 & Z=0 |
LS |
Less than (NS) |
C=0 | Z=1 |
GE |
Greater or equals (WS) |
N=V |
LT |
Less than (WS) |
N!=V |
GT |
Greater than (WS) |
Z=0 & N=V |
LE |
Less or equals than (WS) |
Z=1 | N!=V |
(empty) |
Always (non conditional) |
NS - No sign
WS - With sign
Most of intructions can be executed using conditionals. Ie:
`movle r2, r1
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