Question

Write a Vole program that reverses the contents of the memory cell at address \(0 x 8 C\). (That is, the final bit pattern at address \(0 \times 8 \mathrm{C}\) when read from left to right should agree with the original pattern when read from right to left.)

Short answer

Reverse each bit of the value at \(0x8C\) and store the result back at \(0x8C\).

Step-by-step solution

Load Original Value

Start by reading the value stored at memory address \(0x8C\). Use the LOAD instruction in Vole to transfer this value to the accumulator for manipulation.

Initialize Result Storage

Set up a new memory location to store the reversed value. You can use a memory location such as \(0x90\) for this purpose.

Initialize Loop Counter

If reversing an 8-bit value, prepare a loop counter with the value 8. This will help iterate over each bit of the original value.

Reverse Bits Using a Loop

Use a loop to process each bit of the value from the accumulator. Shift the original value right by 1 bit and examine the least significant bit. Append this bit to the left side of the result at \(0x90\) by shifting it left in \(0x90\).

Update Loop Counter and Continue

Decrement the loop counter with each iteration. Continue the loop until the counter is zero, ensuring each bit of the original value has been processed into the reversed position.

Store Reversed Value

Once all bits have been processed and the result is reversed, transfer the new reversed value from \(0x90\) back to \(0x8C\) using the STORE instruction.

Key concepts

Vole Instruction Set

The Vole instruction set is an essential component when programming in assembly language. This set defines the operations that the Vole system can perform. Think of it as the vocabulary that the machine understands; it includes a finite list of commands such as LOAD, STORE, and SHIFT.
With the LOAD instruction, data is moved from a memory location to the accumulator, holding data temporarily for operations.
STORE is the opposite; it transfers data from the accumulator back to a memory cell. These basic commands are the building blocks for more complex programming structures. Understanding them is crucial to creating efficient Vole programs.

Bit Manipulation

Bit manipulation involves algorithms and techniques to change bit patterns using operations like bit shifting, which are often faster than arithmetic operations. In our exercise, we are dealing with reversing bits. Typically, this involves taking a binary number and flipping its individual digits (0s become 1s and vice versa).
Right and left shift operations are used extensively. A right shift moves every bit in the number to the right, essentially dividing by two. A left shift does the opposite, multiplying by two and placing zeros in the newly created bits. These shifts preserve the original value's binary integrity while altering its positional layout, crucial in reversing bit sequences.

Memory Addressing

Memory addressing is the method used to access data stored in memory locations identified by a unique address, such as hexadecimal values like 0x8C and 0x90 in our scenario. Understanding memory addressing is key to any low-level programming task.
Each address stores particular data or instructions, and effective memory management ensures that programs run smoothly, without overwriting important data unintentionally.
In our case, we use 0x8C for the initial value and 0x90 as temporary storage during bit reversal. Accurate addressing ensures data integrity and correct operation.

Loop Structures

Loop structures in programming allow you to execute a block of instructions repeatedly until a certain condition is met. This concept is fundamental in our bit reversal process. We employ a count-controlled loop, often realized with a decrementing counter, to manage the process.
Each iteration of the loop in our problem performs a bit shift and test operation, moving through the entire bit string until all bits have been processed. Upon each pass, the loop counter decreases by one, and when it reaches zero, the loop exits.
This basic form of loop is essential for tasks that require iterating over array-like structures or manipulating entire bit sequences.