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

2 of 37 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.

3 of 37 CPU Instruction Set ARM7TDMI processor has two instruction sets: the standard 32-bit ARM instruction set a 16-bit THUMB instruction set.

4 of 37 Processor Operating States ARM state which executes 32-bit, word-aligned ARM instructions. THUMB state which operates with 16-bit, halfword-aligned THUMB instructions.

5 of 37 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.

6 of 37 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.

7 of 37 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)

8 of 37 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

9 of 37 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

10 of 37 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.

11 of 37 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

12 of 37 ARM7TDMI core

13 of 37 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

14 of 37 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

15 of 37 THUMB Register Organisation Thumb General registers and Program Counter Thumb Program Status Registers User / System FIQ SupervisorAbortIRQUndefined CPSR sprsr_fiq SPSR_ABTSPSR_SVCsprsr_fiq SPSR_FIQsprsr_fiq SPSR_IRQ CPSR sprsr_fiq SPSR _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

16 of 37 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.

17 of 37 ARM-Thumb differences Unconditional Execution of instruction 2-address format for data processing Less regular instruction formats.

18 of 37 Thumb exception With exception processor is returned to ARM mode. While returning previous mode is restored as SPSR is transferred to CPSR

19 of 37 Thumb Branching Short conditional branches Medium range unconditional branches Long range Subroutine calls Branch to change to ARM Mode

20 of 37 Branch Instruction Formats Condition8-Bit Offset – Bit Offset 1 1 1H11 – Bit Offset HRm0 0 0 B BL BX Rm

21 of 37 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.

22 of 37 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

23 of 37 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 – Bit Immediate

24 of 37 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’)

25 of 37 Instruction formats Rd, Rn, Rm Rd, Rn, # Rd|Rn, Rm|Rs Rd, Rn, # Rd, #

26 of 37 Instructions MOV Rd, # MVN Rd, Rm CMP Rn, # CMP Rn, Rm CMN Rn, Rm TST Rn, Rm

27 of 37 Instruction ADD Rd, Rn, # ADD Rd, # ADD Rd, Rn, Rm ADC Rd, Rm SUB Rd, Rn, # SUB Rd, # SUB Rd, Rn, Rm SBC Rd, Rm NEG Rd, Rn

28 of 37 Instruction LSL Rd, Rm, # LSL Rd, Rs LSR Rd, Rm, # LSR Rd, Rs ASR Rd, Rm, # ASR Rd, Rs ROR Rd, Rs

29 of 37 Instruction AND Rd, Rm EOR Rd, Rm ORR Rd, Rm BIC Rd, Rm MUL Rd, Rm

30 of 37 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, # ADD Rd, SP, # ADD SP, SP, # SUB SP, SP, # Except others donot set condition code bits

31 of 37 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]

32 of 37 Multiple register transfers LDMIA|STMIA Rn!, { } 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.

33 of 37 Stack Mode POP|PUSH { {, 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

34 of 37 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

35 of 37 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.

36 of 37 Summary Thumb ARM mode switching Similarities and differences Thumb exception handling Data processing instructions Data transfer instructions Separate shift instructions Restricted stack operations.

37 of 37 Thank You, Any Questions ?