Question

Which of the following are invalid uses of immediate addressing mode? (a) LDI R20,0x 24 (b) STS \(0 \times 70,0 \times 30\) (c) OUT \(0 \times 20,0 \times 42\)

Short answer

Options (b) and (c) are invalid uses of immediate addressing mode. Option (b) uses an address as an immediate value, which is not allowed, and option (c) may incorrectly use an immediate value for data if 0x42 is not a reference to a register.

Step-by-step solution

Understand Immediate Addressing Mode

Immediate addressing mode is when an instruction's operand is a constant value or an expression that directly provides this value.

Analyze option (a) for validity

Option (a) uses the Load Immediate (LDI) instruction to load the value 0x24 directly into the register R20. This is a valid use of immediate addressing as the operand is a constant value.

Analyze option (b) for validity

Option (b) uses the Store Direct (STS) instruction to store data from a register (implied to be 0x30) to a specific address (0x70). The STS does not accept immediate values as the destination address, so this is an invalid use of immediate addressing. Immediate values can only be source operands.

Analyze option (c) for validity

Option (c) uses the OUT instruction to write data to an I/O register. The addressed I/O register (0x20) and the value to be written (0x42) look like they are being used as immediate values. However, the OUT instruction typically does not support immediate values for the data to be written; it takes a value from a register. Therefore, this would be an invalid use of immediate addressing if 0x42 is not a reference to a register.

Key concepts

AVR Microcontroller

AVR microcontrollers are a type of 8-bit microcontroller developed by Atmel, which is now part of Microchip Technology. These microcontrollers are popular among electronic hobbyists and professionals alike due to their versatility, power efficiency, and ease of use. With their RISC (Reduced Instruction Set Computer) architecture, they are capable of executing most instructions in a single clock cycle. This results in high performance in a small power footprint, making AVR microcontrollers ideal for many embedded system applications.

AVR microcontrollers come with a range of built-in features, such as EEPROM for non-volatile storage, programmable I/O pins, and various interfaces for serial communication. Additionally, they have a comprehensive instruction set that enables efficient manipulation of data and control of peripheral devices. Understanding how to work with these instructions is key to unlocking the full potential of the AVR in design projects.

LDI instruction

The LDI (Load Immediate) instruction is a fundamental operation in the AVR microcontroller's instruction set, which allows loading of an 8-bit constant directly into one of the 32 general-purpose registers (R16-R31). The syntax for the LDI instruction typically looks like LDI Rd, K, where Rd represents the destination register and K the constant to be loaded.

The immediate addressing mode used by LDI enables the setup of registers with known values without the need to transfer the data from memory or another register. For example, LDI R20, 0x24 would load the hexadecimal value 0x24 into register R20. This efficiency supports initialization and control tasks within an embedded application.

STS instruction

Moving on to the STS (Store Direct to Data Space) instruction, it's an AVR command that stores the contents of a register into the direct address space. The STS instruction has a syntax of STS addr, Rr, where addr is the 16-bit address in the data space where contents of the register Rr will be stored.

It's essential to point out that the STS instruction does not accept immediate values as the destination; it always requires a memory address. The immediate value can only be the source operand, which is the data to be stored, not where it's being stored. Consequently, the attempt of using an immediate value for the address, as in option (b) above, is not a valid operation and represents a misunderstanding of the STS instruction's capabilities.

OUT instruction

Lastly, let's delve into the OUT instruction. It serves the purpose of writing data from one of the AVR's internal registers to an I/O space location. The typical syntax for the OUT instruction is OUT A, Rr, in which A represents the address of the I/O space and Rr indicates the register containing the data to be output.

Unlike the LDI instruction, the OUT instruction is not designed to accept an immediate value for the data to be written; instead, it expects a register reference. Therefore, writing an immediate value directly to an I/O space with the OUT instruction is invalid. For correct use, data must be moved to a register first and then written from that register to the I/O space using the OUT instruction. This ensures accurate and efficient management of the embedded system's I/O operations.