CISC processors use complex instructions to complete tasks in fewer lines of code, while RISC processors use only simple instructions that execute in one clock cycle. ARM introduced the Thumb instruction set, where instructions are 16 bits rather than ARM's 32 bits, to reduce memory requirements. Thumb code takes up around 30% less memory than equivalent ARM code. Thumb instructions allow switching between ARM and Thumb modes and provide stack operations like PUSH and POP with 16-bit instructions, improving code density for embedded systems that typically use RISC architectures.

1. THUMB MODE
Varun K
144520
Nitw
ARM

2. CISC Vs RISC
• CISC
• complete a task in as few lines of assembly
• MULT 2:3, 5:2;; "complex instruction.“
• "a = a * b“
• Advantage:compiler has to do very little work,
very little RAM is required to store instructions

3. ARCHITECTURE

4. CISC Vs RISC
• RISC
• RISC processors only use simple instructions that can be
executed within one clock cycle.
• LOAD; PROD; STORE.
• LOAD A, 2:3
LOAD B, 5:2
PROD A, B
STORE 2:3, A
• Because there are more lines of code, more RAM is needed to
store the assembly level instructions.
• The compiler must also perform more work to convert a high-level
language statement into code of this form

5. Why THUMB??
• Most of the embedded processor uses risc
architecture.
• ARM introduced THUMB instruction set to reduce
memory requirements
• ARM7TDMI
• On average, a Thumb implementation of the same
code takes up around 30% less memory than the
equivalent ARM implementation

6. How THUMB is implemented??
• 16 bit instruction –complete shorthand for subset of
32 bit instruction set.
• All 16 bit instruction has 32 bit equivalent
• No special to access cpsr
• Why 2 instruction set?—Expansion

7. Hardware
•Difference between 32 bit and 16 bit equivalent

8. Register sets

9. Register set
• 12 registers common-data can be passed
• Push, pop ==> unique
• monitoring thumb mode-cpsr

10. CODE DENSITY
Arm code Thumb code

11. ARM-THUMB INTERWORKING
• How do you switch from arm to thumb?
• To call a Thumb routine from an ARM routine,
the core has to change state.
• Cpsr 7th bit-state change
• BX,BLX
 BX r0 –processor enters in to thumb mode if
bit0 of the address is set.

12. ARM-THUMB INTERWORKING(cont.)
• BX rm|label
• Illustrating arm-thumb interworking:-

13. OTHER BRANCH INSTRUCTIONS

14. Data Processing Instruction-subset

15. Simple thumb addition

16. Load-Store Instructions

17. Load-Store Instructions

18. STACK INSTRUCTIONS: PUSH,POP

19. Summary
• All Thumb instructions are 16 bits in length.
• Thumb provides approximately 30% better
code density over ARM code.
• The Thumb instruction set includes POP and
PUSH instructions as stack operations.
• There are no Thumb instructions to access the
coprocessors cpsr, and spsr.