06. thumb instructions

technical

1. Embedded System Design Center
Sai Kumar Devulapalli
ARM7TDMI Microprocessor
Thumb Instruction Set

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Objectives
•To understand 16-bit Thumb mode operation of ARM Processor.
•To understand the features of Thumb mode operation and how
Thumb instructions decompress to ARM Mode.
•To know the technique of switching between ARM and Thumb
mode of operations.
•To know the similarities and differences between ARM and
Thumb mode of operation
•To understand exception handling and branching in Thumb
mode.
•To understand operation of data processing instructions and data
transfer instructions in Thumb mode.

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CPU Instruction Set
ARM7TDMI processor has two instruction sets:
•the standard 32-bit ARMinstruction set
•a 16-bit THUMBinstruction set.

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Processor Operating States
ARM state
which executes 32-bit, word-aligned ARMinstructions.
THUMB state
which operates with 16-bit, halfword-aligned THUMB
instructions.

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Thumb Instruction Set
•ARM architecture versions v4T and above define a 16-bit
instruction set called the Thumb instruction set. The
functionality of the Thumb instruction set is a subset of the
functionality of the 32-bit ARM instruction set.
•A processor that is executing Thumb instructions is operating in
Thumb state. A processor that is executing ARM instructions is
operating in ARM state.

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Thumb Instruction Set
•A processor in ARM state cannot execute Thumb instructions,
and a processor in Thumb state cannot execute ARM
instructions. You must ensure that the processor never receives
instructions of the wrong instruction set for the current state.
•Each instruction set includes instructions to change processor
state.
Note: ARM processors always start executing code in ARM state.

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Thumb Instruction Set
•The processor in Thumb mode uses same eight general-purpose
integer registers that are available ARM mode.Some Thumb
instructions also access the PC(ARM register 15),the Link
Register(ARM register 14) and Stack Pointer(ARM register 13).
•When R15 is read,bit[0] is zero and bits[31:1]contain the
PC.when R15 is written,bit[0] is IGNORED and bits[31:1] are
written to the PC.
•Thumb does not provide direct access to the CPSR or any
SPSR.
•Thumb execution is flagged by the T bit(bit[5]) in the CPSR.
T==0 32-bit instructions are fetched(ARM instruction)
T==1 16-bit instructions are fetched(Thumb instruction)

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Thumb applications
In a typical embedded system:
•use ARM code in 32-bit on-chip memory for small speed-
critical routines
•use Thumb code in 16-bit off-chip memory for large non-
critical control routines
Note:
Switching between ARM and Thumb States of Execution
Using BX Instruction

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Thumb applications
For Most Instruction Generated by the Compiler
•Condition Execution is not used.
•Source and Destination Registers are identical
•Only low registers used
•Constants are limited size
•Inline barrel shifter not used

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DATA TYPES
Byte (8-bit):
placed on any byte boundary.
Half-word (16-bit):
aligned to two-byte boundaries.
Word (32-bit):
aligned to four-byte boundaries.

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Features
•Not a complete architecture
•Dynamically decompressed to ARM Instruction
•Fully supported by ARM development tools
•Both entry and exit are done using corresponding BX
Instruction

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ARM7TDMI core

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Mode Switching
•Default entry to exception mode is always ARM
•Explicit entry to Thumb is done using ARM mode BX
Instruction
•Explicit entry back to ARM mode is done using Thumb mode
BX Instruction

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Thumb Programmers Model
•Registers r0 to r7 are accessible (Lo)
•Few instructions require r8 to r15 to be specified
•r13 is used as the stack pointer
•r14 is used as the link register
•r15 is used as the program counter

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THUMB Register Organisation
Thumb General registers and Program Counter
Thumb Program Status Registers
User / System FIQ Supervisor Abort IRQ Undefined
CPSR
sprsr_fiqsprsr_fiqsprsr_fiq SPSR_ABTSPSR_SVCsprsr_fiqsprsr_fiqSPSR_FIQ sprsr_fiqsprsr_fiqsprsr_fiqsprsr_fiqsprsr_fiqSPSR_IRQ
CPSRCPSRCPSRCPSRCPSR
sprsr_fiqsprsr_fiqsprsr_fiqsprsr_fiqsprsr_fiqSPSR _UND
PC
LR
SP
r7
r4
r5
r2
r1
r0
r3
r6
PC_ FIQ
r7
r4
r5
r2
r1
r0
r3
r6
LR_ FIQ
SP_FIQ
PC_ SVC
r7
r4
r5
r2
r1
r0
r3
r6
LR_ SVC
SP_SVC
PC_ ABT
r7
r4
r5
r2
r1
r0
r3
r6
LR_ ABT
SP_ABT
PC_ IRQ
r7
r4
r5
r2
r1
r0
r3
r6
LR_ IRQ
SP_IRQ
PC_ UND
r7
r4
r5
r2
r1
r0
r3
r6
LR_ UND
SP_UND

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ARM-Thumb Similarities
•Load-store architecture
•Support 8-bit byte, 16-bit half-word and 32 bit word data types
with aligned boundaries
•32 bit unsegmented memory.

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ARM-Thumb differences
•Unconditional Execution of instruction
•2-address format for data processing
•Less regular instruction formats.

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Thumb exception
•With exception processor is returned to ARM mode.
•While returning previous mode is restored as SPSR is
transferred to CPSR

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Thumb Branching
•Short conditional branches
•Medium range unconditional branches
•Long range Subroutine calls
•Branch to change to ARM Mode

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Branch Instruction Formats
1 1 0 1Condition 8-Bit Offset
1 1 1 0 0 11 –Bit Offset
1 1 1 H 11 –Bit Offset
0 1 0 0 0 1 1 1 0H Rm 0 0 0
B <cond> <label>
B <label>
BL <label>
BX Rm
1514131211 87 0

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Features
•Different format for each case
•Offset is reduced to 11bit and 8 bit
•Offset is shifted left by 1-bit (to give half-word alignment)
and sign-extended to 32 bits.
•BL is more subtle to give 22-bit offset using link register
for temporary storage
•No direct mapping to ARM instructions as Thumb require
half-word aligned offsets.

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BL Instruction
To allow for a reasonably large offset to the target subroutine
each of these two instructions is automatically translated
by the assembler into a sequence of two 16 bit thumb
instructions
1.H = 10
LR := PC + (sign-extended offset shifted left 12 places);
2.H = 11
PC := LR + (offset shifted left 1 place)
3.LR := address of next instruction

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Software Interrupt Instruction
•Address of next instruction is saved in r14_svc
•CPSR is saved in r14_svc
•Disables IRQ, Clears T bit, Enters Supervisor mode
•PC is forced to 0x08
•8 bit immediate is zero extended to fill the 24-bit field in the
ARM instruction.
•Limits SWIs to first 256 of 16 million ARM SWIs.
1 1 0 1 1 1 1 18 –Bit Immediate

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Data Processing
•Fairly complex instruction formats
•No conditional execution
•Separate shift operations provided, no shifting of second
operand
•All data processing instruction set condition code bits (no need
of ‘S’)

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Instruction formats
•<op> Rd, Rn, Rm
•<op> Rd, Rn, # <imm3>
•<op> Rd|Rn, Rm|Rs
•<op> Rd, Rn, #<sh 5>
•<op> Rd, #<imm 8>

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Instructions
•MOV Rd, #<imm8>
•MVN Rd, Rm
•CMP Rn, #<imm8>
•CMP Rn, Rm
•CMN Rn, Rm
•TST Rn, Rm

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Instruction
•ADD Rd, Rn, #<imm3>
•ADD Rd, #< imm8>
•ADD Rd, Rn, Rm
•ADC Rd, Rm
•SUB Rd, Rn, #<imm3>
•SUB Rd, #< imm8>
•SUB Rd, Rn, Rm
•SBC Rd, Rm
•NEG Rd, Rn

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Instruction
•LSL Rd, Rm, #<#sh>
•LSL Rd, Rs
•LSR Rd, Rm, #<#sh>
•LSR Rd, Rs
•ASR Rd, Rm, #<#sh>
•ASR Rd, Rs
•ROR Rd, Rs

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Instruction
•AND Rd, Rm
•EOR Rd, Rm
•ORR Rd, Rm
•BIC Rd, Rm
•MUL Rd, Rm

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Instruction (using Hi registers)
•ADD Rd, Rm (1 or 2 Hi registers)
•CMP Rn, Rm (1 or 2 Hi registers)
•MOV Rd, Rm (1 or 2 Hi registers)
•ADD Rd, PC, #<imm8>
•ADD Rd, SP, #<imm8>
•ADD SP, SP, #<imm7>
•SUB SP, SP, #<imm7>
Except others donot set condition code bits

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Data Transfer Instruction
•LDR|STR Rd, [Rn, #off5]
•LDR|STR Rd, [Rn, Rm]
•LDRB|STRB Rd, [Rn, #off5]
•LDRB|STRB Rd, [Rn, Rm]
•LDRH|STRH Rd, [Rn, #off5]
•LDRH|STRH Rd, [Rn, Rm]
*Signed operands:
•LDR|STR {S} {H|B} Rd, [Rn, Rm]

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Multiple register transfers
•LDMIA|STMIA Rn!, { <reg list> }
•Rn may be any register among Ro –R7
•Register set can be any subset of R0 –R7 but not base
register ‘Rn’
•Write back to base register is always selected.

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Stack Mode
•POP|PUSH { <reg list> {, R}}
•R13 (sp) is used as base register
•Uses Full Descending Stack
•In addition any subset of Ro-R7 registers LR (lr) may be
included in PUSH instruction and PC (pc)may be included in
POP instruction

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Thumb-ARM Decompression
•Translation from 16-bit Thumb instruction to 32-bit ARM
instruction
•Condition bits changed to ‘always’
•Lookup to translate major and minor opcodes
•Zero extending 3-bit register specifiers to give 4-bit
specifiers
•Zero extending immediate values
•Implicit ‘S’(affecting condition codes) should be explicitly
specified.
•Thumb 2-address format must be mapped to ARM 3-
address format

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Properties
•Thumb code requires 70% of space of ARM code
•Thumb code uses 40% more instructions than the ARM
code
•With 32-bit memory ARM code is 40% faster
•With 16-bit memory Thumb code is 45% faster than ARM
code
•Thumb code uses 30% less external memory power than
ARM code.

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Summary
•Thumb ARM mode switching
•Similarities and differences
•Thumb exception handling
•Data processing instructions
•Data transfer instructions
•Separate shift instructions
•Restricted stack operations.

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Thank You, Any Questions ?