30 Questions
What is the purpose of the code provided?
To copy elements from one memory block to another using LDM and STM instructions
What is the value of the counter variable?
18
What is the purpose of the STMFD SP!, {R4 - R11} instruction?
To save the current values of registers R4 to R11 onto the stack
What is the purpose of the LDMIA R0!, {R4 - R11} instruction?
To load 8 words from the src memory block into registers R4 to R11
What is the purpose of the STMIA R1!, {R4 - R11} instruction?
To store the values in registers R4 to R11 into the dst memory block
What is the purpose of the SUBS R3, R3, #1 instruction?
To subtract 1 from the value in register R3
What happens when an 8-bit or 16-bit value is stored from the register to the memory?
No extension is required
Which type of instructions are used to perform read/write operations in memory?
Load and Store Instructions
When the optional type field is not specified for LDR and STR instructions, how many bits are exchanged between memory and processor registers?
32 bits
In RISC architecture, how are memory access instructions restricted compared to CISC architecture?
Only a few instructions are allowed
In which addressing mode does the register containing the base address get updated before accessing the memory location?
Pre-Indexed Offset Addressing Mode
What do Load/Store instructions require for memory addressing?
Memory addressing
Which type of instructions do Push and Pop belong to?
Stack Memory Access Instructions
What field can be used to make the execution of LDR and STR instructions conditional?
Cond field
Which addressing mode defines updating the base address before accessing a memory location?
Pre-Indexed Offset Addressing Mode
What is the purpose of the Load instruction in memory access?
To bring data from memory to a register
What addressing modes are defined in the Thumb2 instruction set architecture?
Pre-indexed immediate offset addressing, Post-indexed immediate offset addressing
What is the purpose of the PUSH {R7, R8} instruction?
It pushes registers R7 and R8 onto the stack and decrements the stack pointer by 8.
Which direction does the stack grow in?
Downward direction
What is the order in which registers are popped from the stack when using the POP instruction?
The registers are popped in ascending order of their numbers.
Which of the following is a reason why a stack is needed in a computer system?
All of the above.
What is the relationship between the stack pointer (SP) and the last item pushed onto the stack?
SP points to the last item pushed
How does RISC architecture handle Load/Store Instructions in terms of flags?
Load/Store Instructions do not affect flags
What is the purpose of the POP instruction?
To copy words from memory and store them in registers
What is the order in which registers are pushed onto the stack when using the PUSH instruction?
The registers are pushed in the order they are listed in the instruction.
What happens to the stack pointer (SP) register when a data object is pushed onto the stack?
SP decrements by 4
What happens to the stack pointer (SP) register when a data object is popped from the stack?
SP increments by 4
What is the purpose of the PUSH instruction?
To copy data objects from a register or a list of registers onto the stack
What is the purpose of the POP {R0-R7, R12, R14} instruction?
It pops registers R0 through R7, R12, and R14 from the stack and increments the stack pointer by 40.
What is the purpose of storing the return address in the stack during a subroutine call?
To allow the processor to return to the correct location after the subroutine has completed.
Study Notes
Stack Memory Access with PUSH and POP
- The stack is part of main memory, used to store data objects in last-in-first-out (LIFO) buffering format.
- In ARM Cortex-M processor, the stack always operates on 32-bit data.
- The stack pointer contains an address that points to a 32-bit data at the top of the stack.
- As we push data objects onto the stack, the addresses are decremented.
Load and Store Instructions
- Load and Store instructions are used to do a read/write operation in Memory.
- Load is used to take data from a memory location and write that data into a processor register.
- Store is used to store the content of a register into a memory location.
- Load/Store instructions require memory addressing and do not affect the flags.
Immediate Offset Addressing
- Example of immediate offset addressing: LDR R0, [R1] ; Loads R0 from address in R1
- STR R2, [R9, #0x7] ; Stores R2 to a memory location with address R9+7
- Note that R1 and R9 remain unchanged.
Pre-Indexed Offset Addressing Mode
- Syntax: LDR/STR Rt, [Rn, #offset]!
- The register containing the base address is updated before accessing the memory location.
- This updated address is used by the LDR or STR instruction during execution.
- Base address register is updated.
PUSH and POP Instructions
- PUSH instruction copies one or more data objects from a register or a list of registers onto the stack, whose starting address is determined by SP.
- POP instruction copies words from memory and stores them in registers.
- SP decrements (by 4) with each PUSH and increments (by 4) with each POP.
- To push a data object on the stack, the stack pointer is first decremented by 4, and then the 32-bit information is stored at the address specified by SP.
- To pop a data object from the stack, the 32-bit information pointed to by SP is first retrieved, and then the stack pointer is incremented by 4.
Why Do We Need a Stack?
- In subroutine calls, the stack is used to store register values temporarily.
- Registers are saved (pushed to stack) at the start of subroutine, then restored (popped from stack) at the end of the subroutine.
- Each PUSH instruction must have a corresponding POP instruction.
- The stack can be used for parameter passing purposes, if available registers are not enough to pass parameters.
- The stack can be used for declaring local variables.
- The stack can be used for storing the return address.
Test your knowledge on key concepts related to memory access instructions in ARM Cortex-M architecture such as load and store instructions, immediate offset addressing, register offset addressing, and more. This quiz is based on Chapter 6 of the book 'ARM Microprocessor Systems: Cortex-M Architecture, Programming, and Interfacing' by Tahir Muhammad and Kashif Javed.
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