Assembly Language Techniques Programming the MCS51 Microcontroller - - PowerPoint PPT Presentation

Assembly Language Techniques

Programming the MCS51 Microcontroller Background Data Transfer Instructions Data Processing Instructions Program Flow Control Instructions Interrupts

Chatchai Jantaraprim (cj@coe.psu.ac.th)

Program Flow Control Instructions

Branching Instructions Allow microntroller to take different actions at run time In hardware, it modified Program Counter, defined the next instruction to be fetched Normally next immediate following instruction is executed

Braching instruction place different value in Program Counter

Braching includes: Jumps Call/Return

Program Flow Control Instructions

Unconditional Jump Instructions SJMP<rel>

Short jump, 127 byte ahead, 128 byte behind

AJMP<addr11>

Absolute jump, 11-bit (2K Block)

LJMP<addr16>

Long jump, 16-bit (Entire 64K address space)

JMP@A+DPTR

Long jump, from lookup table

• rg 8000horg 8000h

mov C, P1.0start:mov C, P1.0 mov P1.1, Cmov P1.1, C ljmp 8000hsjmp start

Program Flow Control Instructions

Unconditional Jump Instructions (cont.)

80001org8000h 8000 A2902start:movC, P1.0 8002 92913movP1.1, C 8004 0280004ljmpstart 0000=5end 80001org8000h 8000 A2902start:movC, P1.0 8002 92913movP1.1, C 8004 80FA4sjmpstart 0000=5end 80001org8000h 8000 A2902start:movC, P1.0 8002 92913movP1.1, C 8004 01004ajmpstart 0000=5end

Program Flow Control Instructions

Unconditional Jump Instructions (cont.)

90001org9000h 9000 A2902start:movC, P1.0 9002 92913movP1.1, C 9004 0290004ljmpstart 0000=5end 90001org9000h 9000 A2902start:movC, P1.0 9002 92913movP1.1, C 9004 80FA4sjmpstart 0000=5end 90001org9000h 9000 A2902start:movC, P1.0 9002 92913movP1.1, C 9004 01004ajmpstart 0000=5end

Program Flow Control Instructions

Unconditional Jump Instructions (cont.) Indexed Jump jump @a+dptr

... anl a, #3 rl a rl a mov dptr, #jump_table jmp @a+dptr jump_table: ljmp routine_0 nop ljmp routine_1 nop ljmp routine_2 nop ljmp routine_3 ...

Program Flow Control Instructions

Unconditional Jump Instructions (cont.) Short jump and Absolute jump is relocatable, but with limited jump range Long jump is not relocatable, but provides program to jump to anywhere Indexed jump provides a convenient means to construct a jump table

Program Flow Control Instructions

Conditional Jump Instructions JZ<rel> JNZ<rel> JC<rel> JNC<rel> JB<bit>, <rel> JNB<bit>, <rel> JBC<bit>, <rel> CJNEA, direct, <rel> CJNEA, #data, <rel> CJNERn, #data, <rel> CJNE@Ri, #data, <rel> DJNZRn, <rel> DJNZdirect, <rel>

Program Flow Control Instructions

Conditional Jump Instructions (cont.)

delay: mov r7, #0 mov r6, #0 delay0: djnz r6, $ djnz r7, delay0 ret

Program Flow Control Instructions

Call and Return Instructions

Call push PC on stack before branch Return pop PC off stack

Call and return are necessary to implement subroutines. ACALL<addr11>

branch within 2K block

LCALL<addr16>

branch anywhere in 64K

RET

pop PC

RETI

pop PC and reset interrupt hardware

Program Flow Control Instructions

Call and Return Instructions (cont.)

LED_flagequ0Fh

• rg8000h

start: jbLED_flag, LED_OFF setbLED_flag clrP1.0 lcalldelay sjmpstart LED_OFF: clrLED_flag setbP1.0 lcalldelay sjmpstart

Program Flow Control Instructions

Call and Return Instructions (cont.) Computed goto techniques:

• rg8000h

movdptr, #next pushdph pushdpl mova, #81h pushacc clra pushacc ret next: ljmp8000h

• rg8100h