Question

Example the Pep/9 system contains the following four hexadecimal values

register A: 25B6

register X: AE20

Mem[091D]: 0FF0

Mem[0A71]: 301D

If these are the values before each of the following instructions executes, indicate which of these values is changed when each statement is executed, and show the new value. If a register or a memory location shown above is unchanged by the instruction, don't show its contents; only show the item that changed and what the new 16-bit value of that item would be. Note: reset the contents of the registers and memory locations to the given values above before executing each of the following instructions (in other words, these instructions are independent of each other.

1. C1091D
2. D1091D
3. F90A71
4. 69091D
5. 990A71
6. 08

Explain please

Answer 1

Greetings!!

Machine code represents the following instructions:

1. C1091D - represent the instruction LDWA 0X091D,D
2. D1091D - represent the instruction LDBA 0X091D,D
3. F90A71 - represent the instruction STBX 0X0A71,D
4. 69091D - represent the instruction ADDX 0X091D,D
5. 990A71 - represent the instruction ORX 0X0A71,D
6. 08 - represent the instruction NEGA

Execution:

For the purpose of execution the following memory addresses are considered.

0x0007 instead of 0x091D and is loaded with given value 0FF0

0x0009 instead of 0x0A71 and is loaded with given value 301D

Execution screenshot:

Register A and X are loaded with 25B6 and AE20

1. LDWA instruction load one word data from address 0007 to register A and it replaces the content of register A with 0FF0

2. LDBA instruction loads one byte of data from memory address 0007 into A by keeping the 0F in the register A itself. Now A contains 0F0F

3. STBX stores the lower byte of data in the X register to memory address 0009. ie 20 is stored at memory address 0009.

4. ADDX add the content of memory address 0007(0FF0) to its previous content AE20 and generate the answer as BE10.

5. ORX instruction does OR operation with BE10 and the content of address 0009 which is 201D and generate an answer BE1D

6. NEGA instruction negates the content of register A

Hope this helps