In the 8085 microprocessor, which restart (RST) vector is associated with the TRAP interrupt, and what is its vector address?

Introduction / Context:
The Intel 8085 microprocessor supports several hardware interrupts, including TRAP and the maskable interrupts RST 7.5, RST 6.5, and RST 5.5. Each interrupt transfers control to a fixed memory address known as a vector. These addresses often correspond to the locations used by RST instructions. Understanding which vector address is associated with each interrupt is important in low level programming and interrupt driven design. This question focuses on the relationship between TRAP and its internal restart vector.

Given Data / Assumptions:

• The interrupt in question is TRAP, which is a non maskable interrupt in the 8085.
• Restart instructions RST n use addresses of the form 8 multiplied by n, where n may be a half integer in internal notation.
• The TRAP interrupt is vectored to address 0024H in memory.
• We are connecting this address with the symbolic restart number used in documentation.

Concept / Approach:
In the 8085, each RST instruction transfers control to a fixed location at address 8 multiplied by the restart number. For example, RST 3 corresponds to address 0018H and RST 4 corresponds to address 0020H. The TRAP interrupt, which has the highest priority among the hardware interrupts, is vectored to address 0024H. This address corresponds to an internal restart vector often described as RST 4.5, because 4.5 multiplied by 8 equals 36 in decimal, which is 24H in hexadecimal. While programmers do not explicitly use an RST 4.5 instruction, documentation uses this notation to show the relationship.

Step-by-Step Solution:
Step 1: Recall that TRAP is a non maskable interrupt with the highest priority in the 8085.Step 2: Remember that the TRAP vector address is 0024H in memory.Step 3: Use the general rule that the restart vector address is 8 multiplied by the restart number to work backwards.Step 4: Solve 8 multiplied by n equals 24H (which is 36 in decimal) to get n equal to 4.5.Step 5: Conclude that documentation refers to the TRAP vector as RST 4.5, associated with address 0024H.

Verification / Alternative check:
Microprocessor textbooks and training material usually present a table listing the vector addresses of all 8085 interrupts. In such tables, maskable interrupts RST 7.5, RST 6.5, and RST 5.5 correspond to addresses 003CH, 0034H, and 002CH respectively, while TRAP points to 0024H. These addresses align with the 8 multiplied by n pattern, confirming the internal restart number. The special notation RST 4.5 is used to maintain this arithmetic sequence, even though no explicit RST 4.5 instruction exists in the standard instruction set.

Why Other Options Are Wrong:
Option B is incorrect because RST 7.5 refers to a maskable interrupt with its own vector at 003CH, not to TRAP. Option C is wrong because RST 0 corresponds to address 0000H, which is often the reset vector but not the TRAP vector. Option D is incorrect because RST 6.5 refers to a different maskable interrupt with vector 0034H, again not to the TRAP interrupt.

Common Pitfalls:
Students sometimes confuse TRAP with RST 7.5 simply because both have high priority, but only TRAP is non maskable and uses the 0024H vector. Another pitfall is to forget the 8 multiplied by n pattern for restart vectors, which makes it harder to remember the addresses. Keeping a mental map of these vectors and the special role of TRAP makes it easier to write and debug interrupt service routines.

Final Answer:
In the 8085, the TRAP interrupt is associated with RST 4.5 and uses the vector address 0024H, which is described in option A.