F28PL1 Programming Languages
Lecture 2: Assembly Language 1
CISC & RISC
•
•
•
•
CISC: complex instruction set computer
original CPUs very simple
poorly suited to evolving high level languages
extended with new instructions
– e.g. function calls, non-linear data structures, array bound
checking, string manipulation
• implemented in microcode
– sub-machine code for configuring CPU sub-components
– 1 machine code == > 1 microcode
• e.g. Intel X86 architecture
CISC & RISC
• CISC introduced integrated combinations of features
–
–
–
–
e.g. multiple address modes, conditional execution
required additional circuitry/power
multiple memory cycles per instruction
over elaborate/engineered
• many feature combinations not often used in practise
• could lead to loss of performance
• better to use combinations of simple instructions
CISC & RISC
• RISC: reduced instruction set computer
• return to simpler design
• general purpose instructions with small number of
common features
– less circuitry/power
– execute in single cycle
• code size grew
• performance improved
• e.g. ARM, MIPS
ARM
• Advanced RISC Machines
• UK company
• originally Acorn
– made best selling BBC Microcomputer in 1980s
• world leading niche processors
– 2009: 90% of all embedded processors
– mobile phones, media players, games, peripherals etc
– e.g. Nokia, iPod, iPad, Blackberry, Nintendo
ARM
• 32 bit RISC processor architecture family
• many variants
• general reference in library:
– W. Hohl, ARM Assembly Language, CRC Press, 2009
– ARM7TDMI
• ARM does not make chips
• licences the IP core (intellectual property) logic
design to chip manufacturers
Cortex-M3
• ARM CPU in STM32-Discovery board
• Thumb2 instruction set
– based on ARMV7
– subset of full ARM instructions
– 16 bit length instructions
• Cortex-M3 manual
• http://infocenter.arm.com/help/topic/com.arm.doc.dui05
52a/DUI0552A_cortex_m3_dgug.pdf
MDK-ARM
• ARM development environment for:
– C & assembler
– many ARM-based systems
• includes simulator
• free restricted download via:
• http://www.keil.com/arm/mdkbasic.asp
• MDK-ARM/Cortex-M3 guide
• http://www.st.com/internet/com/TECHNICAL_RESOURCES
/TECHNICAL_LITERATURE/USER_MANUAL/CD00283786.pd
f
Cortex-M3 memory map
• linear 4GB address
space
• modified Harvard
architecture in CPU
0.5GB
1GB
0xFFFFFFFF
Vendor Specific
External Peripheral Bus 0xE0100000
Internal Peripheral Bus 0xE0040000
0xE0000000
External Device
0xA0000000
1GB
External RAM
0.5GB
Peripheral
0.5GB
SRAM
0x60000000
0x40000000
0x20000000
0.5GB
Code
0x00000000
Memory
• linear addressing
• 2 descending stacks
– main stack
– process stack – used by OS
• all operations based on registers
• must move data between registers & memory
Registers
• 16 * 32 bit registers
• working registers
– R0-R7 – low registers
– R8-R12 – high registers
• R13 – stack pointer (SP)
• R14 - link register (LR)
• R15 – program counter (PC)
• PSR – program status register
– bits for negative (N), zero (Z), carry (C), overflow (V)
Data formats
• word
– 32 bits
– long
• half word
– 16 bits
– int
• byte
– 8 bits
– char
Programming approach
• in high level programming:
– identify variables & types
– compiler maps these to memory & data representation
– use of memory v registers invisible
• in low level programming
– must make explicit use of memory & registers
– must choose data representation
• registers much faster than memory
• try to map variables to registers
• what if more variables than registers...?
Example: sum of 1st 10 integers
• we write:
• C compiler generates:
#define MAX 11
int s;
int n;
n = 0;
while(n<MAX)
{ s = s+n;
n = n+1;
}
MOVS
MOVS
B
LOOP ADD
ADDS
TEST CMP
BLT
r2,#0x00
r1,#0x00
TEST
r2,r2,r1
r1,r1,#1
r1,#0x0A
LOOP
Example: sum of 1st 10 integers
• we write:
• C compiler generates:
#define MAX 11
int s;
int n;
n = 0;
while(n<MAX)
{ s = s+n;
n = n+1;
}
MOVS
MOVS
B
LOOP ADD
ADDS
TEST CMP
BLT
r2,#0x00
r1,#0x00
TEST
r2,r2,r1
r1,r1,#1
r1,#0x0A
LOOP
Example: sum of first 10 integers
• s == r2 - register 2
• n == r1 – register 1
MOVS
r2,#0x00
• move hexadecimal constant 0x00 to register 2
• set condition code flags
MOVS
r1,#0x00
• move hexadecimal constant 0x00 to register 2
• set condition code flags
B
TEST
• branch to label TEST
Example: sum of first 10 integers
LOOP ADD
r2,r2,r1
• label LOOP
• put in r2 sum of r2 and r1
ADDS
r1,r1,#1
• put in r1 sum of r1 and decimal constant #1
• set condition code flags
Example: sum of first 10 integers
TEST CMP
r1,#0x0A
• label TEST
• compare r1 and hexadecimal constant #0x0A == 10
• set condition code flags
BLT
LOOP
• branch to label LOOP if flags indicate “less than”
Instruction format
mnemonic operands
– general format
– operands depend on instruction
mnemonic
– meaningful short form
– reminds us what instruction does
• ARM uses {...} for options
• Rd or Rm == register -r0..r15
Operands
• first operand usually rd
• Operand2 == flexible 2nd operand
– constant
– Rm
• constant == 8 hexadecimal digits
• constant == decimal
– size of decimal depends on instruction
• #imm16 == 0-65535 == 16 bit
MOV: move
MOV Rd, Operand2 
Rd = Operand2
• don’t update condition code flags
MOVS Rd, Operand2
• as MOV but update condition code flags
MOV Rd, #imm16 
Rd = imm16
ADD: addition
ADD Rd, Rn, Operand2  Rd = Rn+Operand2
ADD Rn, Operand2  Rn = Rn+Operand2
ADDC
• add with carry
• like ADD + carry flag
ADDS/ADDCS
• like ADD/ADDC but set condition code flags
SUB: subtraction
SUB Rd, Rn, Operand2  Rd = Rn-Operand2
SUB Rn, Operand2  Rn = Rn-Operand2
SUBC
• subtract with carry
• like SUB -1 if carry flag not set
SUBS/SUBCS
• like SUB/SUBC but set condition code flags
CMP: compare
CMP Rn, Operand2
• subtract Operand2 from Rn BUT ...
• ... do not modify Rn
– otherwise same as SUBS
• update Z, N, C, V flags
CMN RN, Operand2
• add Operand2 to Rn BUT ...
• ... do not modify Rn
• otherwise same as ADDS
• update Z, N, C, V flags
Flags
• N – 1 if result <0; 0 otherwise
• Z – 1 if result =0; 0 otherwise
• C – 1 if result led to carry; 0 otherwise
– i.e. X+Y > 232
– i.e. X-Y >= 0
Flags
• V – 1 if result led to overflow; 0 otherwise
• (-) == negative
• (+) == positive
– i.e. (-)X+(-)Y > 0
– i.e. (+)X+(+)Y < 0
– i.e. (-)X-(+)Y > 0
– i.e. (+X)-(-Y) < 0
B: branch
B label
• branch to label
• i.e. reset PC to address for label
Bcond label
• branch on condition to label
condition
suffix
EQ == equal
NE == not equal
CS/HS == carry set/ higher or same - unsigned
CC/LO == carry clear/lower - unsigned
flags
MI == negative
PL == positive or 0
VS == overflow
N=1
N=0
V=1
VC == no overflow
V=0
Z=1
Z=0
C=1
C=0
condition
suffix
HI == higher - unsigned
LS == lower or same - unsigned
GE == greater than or equal - signed
LT == less than - signed
flags
GT == greater than - signed
LE == less than or equal, signed
AL == any value
Z=0 & N=V
Z=1 & N!=V
default if not
cond
C=1 & Z=0
C=0 or Z=1
N=V
N!=V
Label
• identifier
• can precede any line of assembler
• represents the address of:
– instruction
– literal data
• relative to program counter (PC)
• turned into PC + offset in machine code
NOP: no operation
NOP
• do nothing
• use for padding/layout
• no cost
Layout
• ARM assembly language is layout sensitive
• must precede instruction with a tab
• for labelled instruction, use tab immediately after
label
• can’t use equivalent number of spaces
Example: multiply by adding
• m == x*y
int x;
int y;
int m;
x = 8;
y = 10;
m = 0;
while(y!=0)
{ m = m+x;
y = y-1;
}
• x == R1; y == R2; m == R3
MOV R1,#0x08
MOV R2,#0x0A
MOV R3,#0x00
LOOP CMP R2,#0x00
BEQ DONE
ADD R3,R1
SUB R2,#0x01
B LOOP
DONE NOP
Register name definition
name RN expression
• expression  0..15
• name renames the
register
e.g.
x
y
m
RN
RN
RN
1
2
3
MOV x,#0x08
MOV y,#0x0A
MOV m,#0x00
LOOP CMP y,#0x00
BEQ DONE
ADD m,x
SUB y,#0x01
B LOOP
DONE NOP
Constant name definition
name EQU expression
• name is replace with
value from expression
e.g.
x
RN 1
y
RN 2
m
RN 3
xinit
EQU 8
yinit
EQU 10
MOV x,#xinit
MOV y,#yinit
MOV m,#0x00
LOOP CMP y,#0x00
BEQ DONE
ADD m,x
SUB y,#0x01
B LOOP
DONE NOP
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F28PL2 Programming Languages