Instruction Set Architectures
• Early trend was to add more and more instructions to new
CPUs to do elaborate operations
–VAX architecture had an instruction to multiply
polynomials!
• RISC philosophy (Cocke IBM, Patterson, Hennessy,
1980s):
Reduced Instruction Set Computing
–Keep the instruction set small and simple; makes it
easier to build fast hardware.
–Let software (compiler) do complicated operations by
composing simpler ones.
• ARM is RISC
Chapter 2: The Programmer’s Model
• Concerned with the features that are available to you
from a high level, e.g.
– Where data can be stored
– What happens when the machine is given an invalid
instruction
Data Types:
• Byte:
8 bits
• Halfword:
16 bits
• Word:
32 bits
Processor Modes
ARM has seven basic operating modes:
• User:
unprivileged mode under which most tasks run
has limited access to the hardware (non-privileged)
whereas all other modes have full access (privileged) to
the CPU resources.
• FIQ:
entered when a high priority (fast) interrupt is raised
• IRQ:
entered when a low priority (normal) interrupt is raised
• Super:
entered on reset and execution of software interrupt
instruction
• Abort:
used to handle memory access violations
• Undef:
used to handle undefined instructions
• System: privileged mode using the same registers as user mode
Processor Modes
• We will mainly use User mode. Other modes much less
important for this class.
• For now, only concerned with r0-r12; treat these as
registers that can be used to store any variable.
ARM Registers
Register – internal CPU hardware device that stores binary
data; can be accessed much more rapidly than a
location in RAM
ARM has
13 general-purpose registers R0-R12
1 Stack Pointer (SP) – R13
1 Link Register (LR) – R14 holds the caller’s return address
1 Program Counter (PC) – R15
1 Current Program Status Register (CPSR)
ARM Registers
• ARM processors, with the exception of ARMv6-M and
ARMv7-M based processors, have a total of 37 or 40
registers depending on whether the Security Extensions
are implemented.
• registers are arranged in partially overlapping banks.
• There is a different register bank for each processor
mode.
• The banked registers give rapid context switching for
dealing with processor exceptions and privileged
operations.
• Additional registers are available in privileged software
execution.
ARM Registers
Additional registers in ARM processors, with the exception
of ARMv6-M and ARMv7-M, are:
2 supervisor mode registers for banked SP and LR
2 abort mode registers for banked SP and LR
2 undefined mode registers for banked SP and LR
2 interrupt mode registers for banked SP and LR
7 FIQ mode registers for banked R8-R12, SP and LR
2 monitor mode registers for banked SP and LR
6 Saved Program Status Register (SPSRs), one for each
exception mode.
The ARM Register Set
Current Visible
Visible Registers
Registers
Current
Abort
Mode
SVC
Undef
Mode
Mode
FIQ
User
IRQMode
Mode
Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r13 (sp)
(sp)
r13
r14 (lr)
(lr)
r14
r15 (pc)
Banked
Banked
Bankedout
out
outRegisters
Registers
Registers
User
FIQ
IRQ
SVC
Undef
r8
r9
r10
r11
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
Abort
r12
r13 (sp)
r14 (lr)
r13 (sp)
r14 (lr)
r13 (sp)
r14 (lr)
r13 (sp)
r14 (lr)
r13 (sp)
r14 (lr)
spsr
spsr
spsr
spsr
spsr
cpsr
spsr
spsr
Processor Status Register(PSR)
Contains: Condition flags that are set by arithmetic and
logical CPU instructions and used for conditional execution
31
30
29
28
27
26
25
24
23
22
21
29
19
N Z C V Q
17
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
GE
Greater than or
equal to
Mode bits
Data
Endianness
bit
Sticky Overflow
Overflow
Carry/Borrow/Extend
Zero
Negative/Less than
Processor Status Register(PSR)
• The N, Z, C, and V bits are the condition code flags.
• Flags are set by arithmetic and logical CPU instructions
and used for conditional execution
• The processor tests these flags to determine whether to
execute a conditional instruction.
N – negative / less than
Z – zero
C – carry / borrow / extend
V – overflow
Processor Status Register(PSR)
– Q sticky overflow. set to 1 when saturation occurs
during QADD, QDADD, QSUB or QDSUB, or the result
of SMLAxy or SMLAWx overflows 32-bits
The Q flag is sticky in that, when an instruction sets it,
this bit remains set until an MSR instruction writing to
the CPSR explicitly clears it.
Instructions cannot execute conditionally on the status
of the Q flag.
Processor Status Register(PSR)
Also Contains:
• Interrupt enable/disable flags for standard (IRQ) and fast
(FIQ) interrupts.
• Thumb bit set on Thumb Instruction execution.
• Mode bits that indicate the current operating mode.
• The PSR can be accessed by the MSR or LDM
instructions.
• Each operation mode has a shadow register for the current
state of the PSR. The shadow registers are called SPSR
(saved processor status register) whereas the CPSR holds
the current state.
ARM Instructions
• ARM instructions are written as an operation code
(opcode), followed by zero or more operands
• Operands may be constants, registers, or memory
references
ARM Instructions
• Instruction syntax:
opcode{cond}{flags} Rd, Rn, operand2
where:
– {cond} is an optional two-letter condition, e.g. EQ
– {flags} is an optional additional flag, e.g. S
– Rd is the destination register
– Rn is the first source register
– Operand2 is a flexible second operand
• Syntax is rigid (for the most part):
–1 operator, 3 operands
–Why? Keep Hardware simple via regularity
ARM Instructions
Note:
• Operand2 is a flexible second operand to most
instructions
– it is passed through the barrel shifter (a functional
unit that can rotate and shift values)
– it can take one of three forms:
o Immediate value: an 8-bit number rotated right
by an even number of places
o Register shifted by a value: a 5-bit unsigned
integer shift.
o Register shifted by a register: the bottom 8 bits
of a register.
ARM Instructions
Examples of Operand2
• Immediate values
o add r0, r1, #3
o mov r0, #15
o mov r1, #0x12
• Register shifted by a value
o mov r0, r1, lsl #4
o orr r1, r1, lsr #10
• Register shifted by a register
o cmp r1, r2, lsl r0
o add r5, r3, ror r0
ARM Comments
• One way to make your code more readable is to use
comments!
• The at symbol, @, is used for ARM comments in
QEMU
–anything from @ to end of line is a comment and
will be ignored
• The block comment, /* comment */, is also available
Immediates
• Immediates are numerical constants.
• They appear often in code, so there are ways to
indicate their existence
• Add Immediate:
/* f = g + 10
(in C)
*/
ADD r0, r1, #10 @ (in ARM)
where ARM registers r0, r1 are associated with C
variables f, g
• The second operand is a #number instead of a
register.
Move instructions
• mov Rd
• mvn Rd
Operand2
0xFFFFFFFF EOR Operand2
• Examples
mov r0, #15
mov r0, r1
Compare instructions
• cmp – compare
o Flags set to result of (Rn – Operand2)
• cmn – compare negative
o Flags set to result of (Rn + Operand2)
• tst – bitwise test
o Rd := Rn or Operand2
• teq – test for equivalence
o Rd := Rn and not Operand2
Compare instructions
• Comparisons produce no results – they just set condition
codes.
• Ordinary instructions will also set condition codes if the
“S” bit is set. The “S” bit is implied for comparison
instructions.
• Examples of compare instructions
cmp r0, r1
cmp r0, #10
tst r1, #1
teq r0, 41
Logical instructions – Chapter 7 (pp 97 – 98)
•
•
•
•
and – logical and
eor – exclusive or
orr – logical or
bic – bitwise clear
• Examples
and r2, r0, r1
eor r2, r2, #1
Rd
Rd
Rd
Rd
Rn and Operand2
Rn eor Operand2
Rn or Operand2
Rn and not Operand2
@ r2 = r0 & r1
@ r2 = r0 ^ 1
Arithmetic instructions (p. 104)
•
•
•
•
•
•
add
adc
sub
sbc
rsb
rsc
Rd
Rd
Rd
Rd
Rd
Rd
Rn + Operand2
Rn + Operand + Carry
Rn – Operand2
Rn – Operand2 – not(Carry)
Operand2 – Rn
Operand2 – Rn – not(Carry)
• Examples
add r2, r0, r1
sub r2, r0, r1
@ r2 = r0 + r1
@ r2 = r0 – r1