TK2633 Lecture6 Control Instructions

Ass Prof Dr Masri AyobAss Prof Dr Masri Ayob

TK 2633:Microprocessor & Interfacing

Lecture 6: Control Instructions

April 8, 2023 Prepared by: Dr Masri Ayob 2

OBJECTIVES

Explain the operation of the unconditional and conditional jump instructions.

Describe the flags tested by each conditional jump instruction.

Explain the operation of the unconditional and conditional call and return instructions.

Show how the stack functions when used by the call and return instructions.

Describe the operation of the instructions: NOP, RST, STC, CMC, RIM, SIM, and HLT.


Control Instructions

Allow computers to make decisions and change the flow of the programme based on the results outcome.

Two main forms: JUMP instructions:

Allow programme to jump to any memory locations.

CALL instructions Allow a group of instructions (subroutine) to

be reused by the program in many different places.


UNCONDITIONAL JUMP INSTRUCTIONS

This is a three-byte instruction that allows the programmer to jump over unused portions of the memory.



The JMP instruction uses direct addressing, and the PCHL instruction uses register indirect addressing.

This causes the next program instruction to execute at the address stored with the JMP instruction.

The PCHL instruction causing the microprocessor to jump to the memory location addressed by the HL register pair.




CONDITIONAL JUMP CONDITIONAL JUMP

Conditional JUMP instructions: Allow the programmer or programme to make a choice

based on conditional terms. A condition is tested by the microprocessor to decide

whether a jump occurs. The conditions tested by the conditional jumps are the

same conditions held in the flag bits. The terms are :

Zero / not zero Carry set / cleared Positive / minus Parity odd / even


CONDITIONAL JUMP INSTRUCTIONS






1. Write a short program that tests the contents of memory location F000H. If it contains a positive number, jump to location F200H; if negative number, jump to location F400H; and if it contains a zero, jump to location F800H.

2. Write a short program that tests the contents of memory location F000H. If it contains a number >20H, jump to location F200H; if it contains a number <20H, jump to location F400H; and if it contains a number =20H, jump to location F800H.

EXAMPLES


A subroutine is a short sequence of instructions that performs a single task.

One advantage of using a subroutine is a significant savings of memory space.

Subroutines also simplify the task of writing a program because subroutines only appear in a program once, but are used often.

CALL instruction allows the programmer to use (link to) a subroutine.

When the 8O85 executes a CALL instruction, two events occur:

The contents of the PC are pushed onto the stack, and The program continues at the address stored with the

CALL instruction. The CALL instruction is a combination of the PUSH and the JMP instructions.

SUBROUTINES


SUBROUTINES


SUBROUTINES The RET(return) instruction returns to the main program at

the instruction that follows the CALL. This can be accomplished because the address of this

instruction is stored on the stack because the CALL placed it there as a return address.

The RET command POPs a number from the stack and places it into the program counter.


SUBROUTINES STACK(before)CALL COMP

??

1000

Note: Let initial SP=1000H

FFFF

STACK

2005 FFFE

AFTER CALL COMP

REGISTERS01A2040PC

FFFESP


Write a short program that ADD the contents of memory location F000H and F001H. If the result >20H, CALL subroutineA; If the result =20H, CALL subroutineB; else CALL subroutineC.

EXAMPLES


These are special unconditional CALL instructions, because they call a subroutine at a fixed location in the memory instead of a variable location as addressed by the CALL instruction. E.g. RST 2

This instruction calls the subroutine at memory location 0010H, i.e. RST 2 CALL 0010H

RST N CALL Nx8

The Restart Instructions


The Restart Instructions


MISCELLANEOUS INSTRUCTIONS Program execution is stopped by the HLT (halt) instruction.

Execution only continues after a HLT instruction by activating reset or have an interrupt occur.

Both the reset and the interrupt must come from the external hardware.

The use of this command must be reserved for special purposes such as catastrophic system failure.


MISCELLANEOUS INSTRUCTIONS An interrupt is a hardware-initiated subroutine call, that

interrupts the currently executing program. Whenever the hardware interrupts the microprocessor, it

calls a subroutine that services the interrupt. This special subroutine is called an interrupt service subroutine (ISR).


MISCELLANEOUS INSTRUCTIONS The RIM and SIM instructions read or write the SID and

SOD pins. The TRAP input is a maskable input that cannot be

affected by the interrupt control instructions. The interrupt control instructions affect the remaining

four inputs (RST 7.5, RST 6.5, RST 5.5, and INTR).


MISCELLANEOUS INSTRUCTIONS Whenever an ISR takes effect, all future interrupts

are disabled (except TRAP). This is why a special instruction (El) reenables the

interrupt inputs. The EI instruction enables INTR and all the interrupt

inputs that are unmasked. The Dl instruction disables all the interrupt inputs

except the TRAP. (Note: TRAP disables other interrupts when accepted by the microprocessor).


SIM INSTRUCTIONS The SIM instruction (set interrupt masks) enables or disables

the RST 7.5, RST 6.5, and RST 5.5 pins. This instruction also controls the SOD (serial output data) pin

on the 8085 and resets the edge-triggered RST 7.5 input. Table 6-8 lists the bits of the accumulator before a SIM and

their effect on the operation of the machine.


SIM INSTRUCTIONS E.g. to set the SOD pin on the 8085, SOD bit (of the acc.) must

equal to ‘1’ and this follows the execution of the SIM instruction.

To change the masks for the RST pins, set the MSE bit and then place a 0 or 1 in each mask bit. A mask of 1 turns the corresponding interrupt input off, and a mask of 0 turns it on.

These instructions enable the RST 6.5 pin and disable the RST 7.5 and RST 5.5 pins. After the mask bit enables the pin, the El instruction must be executed to turn on the RST pin.


DIM INSTRUCTIONS The RIM instruction (read interrupt mask) reads the

SID pin (serial input data), the masks, interrupt pins, and the interrupt enable status.

The RIM instructions will read the information listed in Table 6-9 into the accumulator.


DIM INSTRUCTIONS

The IE bit of the accumulator shows whether El or DI has been executed most recently. This bit also changes whenever an interrupt takes

effect, because interrupts always clear IE disabling future interrupts. .


SUMMARY Program control instructions allow a program to jump around

unused sections of the memory and allow the program to test the flag bits to make decisions.

The unconditional jump (JMP) is a three-byte instruction that causes program execution to continue at the memory address stored with the instruction.

The conditional jump instructions allow the flags (Z, C, S, and P) to be tested. If the outcome of the test is true, a jump occurs; if the outcome is false, the next sequential instruction in the program executes.

Subroutines are short programs that perform one task, end with a return instruction, and can be used often from another program.

The CALL instruction links to a subroutine. It does this by pushing the contents of the program counter onto the stack and jumping to the memory location stored in bytes 2 and 3 of the instruction.


SUMMARY

The return address is the contents of the program counter placed on the stack by a CALL instruction. The return address is removed from the stack by the return (RET) instruction, which places it back into the program counter from the stack.

Conditional call and return instructions work like the conditional jump instruction. If the condition tested is true, the call or return occurs, and if false, the next sequential instruction executes.

Restarts (RST) are one-byte call instructions. The location called by a RST can be determined by multiplying the restart number times 8. For example, RST 5 is equivalent to a CALL 0028H.

The NOP instruction performs no operation and sometimes appears in time- delay software because it takes about 2 us to do nothing.


SUMMARY

The STC and CMC instructions set carry and complement carry, respectively.

The HLT (halt) instruction halts execution until either a system reset or an interrupt.

The 8085A contains five interrupt inputs (TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR) and two serial data pins (SID and SOD).

The RIM (read interrupt masks) and SIM (set interrupt masks) instructions control the interrupt structure of the 8085A and the serial data pins.

El (enable interrupts) and DI (disable interrupts) turn all the interrupts on and off except TRAP, which can never be disabled. The RST pins may still be disabled after an El, if the mask bits are set.


Thank youQ&A

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TK2633 Lecture6 Control Instructions