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Machine organization Question to be answered: Which instruction (provide a memory address) is executed after instruction at location x4002is executed?

Machine organization Question to be answered: Which instruction (provide a memory address) is executed after instruction at location x4002is executed?

Database System Concepts
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
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Machine organization

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Which instruction (provide a memory address) is executed after instruction at location x4002is executed?

## Echo Service Routine in LC-3 Assembly ### Objective This example demonstrates how to define a service routine starting at memory location `x4000` using the LC-3 assembly language. The routine reads a character from the keyboard and echoes it back to the screen. ### Assembly Code ```assembly .ORIG x4000 ; Start at memory location x4000 ST R7, SaveR7 ; Save the current value of R7 GETC ; Get a character from the keyboard OUT ; Output the character to the screen LD R7, SaveR7 ; Restore the original value of R7 RET ; Return from the routine SaveR7 .FILL x0000 ; Define storage for saving R7 ``` ### Explanation 1. **.ORIG x4000**: - This directive specifies the starting address of the program, which is `x4000`. 2. **ST R7, SaveR7**: - This instruction stores the current value of register `R7` into memory location `SaveR7`. - `SaveR7` is defined at the end of the routine to hold the value temporarily. 3. **GETC**: - This instruction reads a character from the keyboard and stores it in register `R0`. 4. **OUT**: - This instruction outputs the character in register `R0` to the screen. 5. **LD R7, SaveR7**: - This instruction loads the value from `SaveR7` back into register `R7`. 6. **RET**: - The `RET` instruction returns control to the program that called this routine by using the value in register `R7`. 7. **SaveR7 .FILL x0000**: - This directive allocates a memory location and initializes it with `x0000`. It is used to save the value of register `R7`. ### Summary This routine demonstrates basic input/output operations in LC-3 assembly language. It illustrates saving and restoring registers to ensure that the routine can be safely called without affecting the rest of the program. Understanding and implementing such routines are fundamental for handling I/O operations in low-level programming.
Transcribed Image Text:## Echo Service Routine in LC-3 Assembly ### Objective This example demonstrates how to define a service routine starting at memory location `x4000` using the LC-3 assembly language. The routine reads a character from the keyboard and echoes it back to the screen. ### Assembly Code ```assembly .ORIG x4000 ; Start at memory location x4000 ST R7, SaveR7 ; Save the current value of R7 GETC ; Get a character from the keyboard OUT ; Output the character to the screen LD R7, SaveR7 ; Restore the original value of R7 RET ; Return from the routine SaveR7 .FILL x0000 ; Define storage for saving R7 ``` ### Explanation 1. **.ORIG x4000**: - This directive specifies the starting address of the program, which is `x4000`. 2. **ST R7, SaveR7**: - This instruction stores the current value of register `R7` into memory location `SaveR7`. - `SaveR7` is defined at the end of the routine to hold the value temporarily. 3. **GETC**: - This instruction reads a character from the keyboard and stores it in register `R0`. 4. **OUT**: - This instruction outputs the character in register `R0` to the screen. 5. **LD R7, SaveR7**: - This instruction loads the value from `SaveR7` back into register `R7`. 6. **RET**: - The `RET` instruction returns control to the program that called this routine by using the value in register `R7`. 7. **SaveR7 .FILL x0000**: - This directive allocates a memory location and initializes it with `x0000`. It is used to save the value of register `R7`. ### Summary This routine demonstrates basic input/output operations in LC-3 assembly language. It illustrates saving and restoring registers to ensure that the routine can be safely called without affecting the rest of the program. Understanding and implementing such routines are fundamental for handling I/O operations in low-level programming.
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