Question

1. Show an instruction schedule for a the instruction "STORE" with syntax "STR Rx, Ry" that will store the contents of Ry into the memory location Rx (i.e., Memory[Rx] <= Ry).
2. Show an instruction schedule for the instruction "SUBTRACT" with syntax "SUB Rx, Ry" that will subtract the value of Ry from Rx and place the result in Rx (i.e., Rx <= Rx - Ry).

Answer 1

(1) Processor execute any instruction through three basic process: 1. Fetch the instruction 2. Decode 3. Execute STORE instruction is the instruction used in Acorn RISC machine (ARM) architecture. It is used to access the memory locations from ARM processor The basic work of STORE instruction is to store the content of Register into an address or location of a memory The Store instruction can be performed either in immediate addressing mode Register addressing mode Here, discuss only register addressing mode. The syntax for STORE instruction using Register addressing mode is, STR Rx, Ry Here, the content of register R is an 8-bit address in which content of Ry gets stored The STR instruction is 4-byte instruction. or Consider the Figure Here, register R contains 00001011 (0B H) and register Ry contains 00001000 08 H) ( Registers Memory Rx0 0 00 1 011 02 00001001 09 H Ry0 0 0 0 1 |0 0 0 00001010 03 OA H 04 00001011 OB H

Execute the instruction STR Rx, Ry The instruction gets executed through following steps: 1. During the first cycle, the processor will fetch the corresponding code of the opcode (STR) 2. The instruction Decoder, then, decode the opcode and release the required gate signals for the corresponding code. 3. During next cycle, processor reads the data in register R and points the corresponding location of memory Here, processor reads data OBH and points the location OB H in the memory address. 4. In the next cycle, the processor sends the content of register Ry to the pointed location of memory address Here, processor sends data 08 H to the pointed location OB H of memory address This process is shown in Figure below Registers Memory Rx 0 0 001 | 0 1|1 Ry0 0001 0 0 0 00001001 00001010 02 09 H 03 OA H 00001011 08 OB H Therefore, the data at register Ry has been successfully stored in memory location pointed by register R.

(2) SUBTRACT instruction is used to calculate the difference between two numbers (group of eight bits in binary) stored in two registers. The syntax for Subtract instruction is, SUB Rx, Ry This syntax of subtract does not consider the carry Syntax SBC R, Ry does the subtraction with carry. Here, the content of register Rx and register Ry is an 8-bit address. This instruction calculates the difference between content of register R and register Ry, treating content of register R as minuend and content of register subtrahend Ry as The result then saved in register Rx. Consider the Figure Here, register Rx contains 00001011( OB H) and register R, contains 00001000 (08 H) Registers R 0 0 00 1 | 0|1|1 RyO 0 00 100 0

Execute the instruction SUB Rx, Ry The instruction gets executed through following steps: 1. During the first cycle, the processor will fetch the corresponding code of the opcode (SUB) 2. The instruction Decoder, then, decode the opcode and release the required gate signals for the corresponding code 3. During next cycle, processor reads the data in register Ry and calculate the 2's compliment of the content in register Ry Here, processor reads data 08H and calculate it's 2's compliment 4. In the next cycle, the processor adds the content of register R with the 2's compliment of content in register Ry. The resultant sum, then overwrite the data in register R Here, after execution of instruction, register R Contains the data 03 H The status of both register after executing the instruction is shown in figure below Registers Rx0 0 00 0 01|1 RYO 0 0 0 1 |0|0|0 Therefore, the subtraction of content in register Ry and register Rs has been done and the difference stored in register R. Note In 8085 microprocessors, the syntax for STORE instruction is STA R Ry. The syntax for SUBTRACT instruction is SUB Ry In 8085 microprocessors, all the process executes in register Accumulator.