Learn 8051

A collection of 8051 EdSim51 programs used to understand the 8051 architecture.

Understanding Stack Pointers

understanding-stack.asm shows the behaviour of 8051 stack pointer.

Path from C file to 8051 byte code

First install sdcc (a small cross compiler for various microcontrollers, http://sdcc.sourceforge.net/) on your machine. For Windows 10, pre compiled binaries were used. Optionally, add SDCC to PATH in order to run sdcc from any arbitrary path on your machine.

Take a look of the example .c file - we will use its logic to examine the assembly and byte code on the 8051:

// addition.c
#include <8051.h>

void main(void)
{   
    for (int i=0; i<3; i++){
        char x = P1;
        x = x - 5;
        P1 = x;
    }
}

Let's compile it:

sdcc .\from-c-to-opcode\addition.c -o .\from-c-to-opcode\

After this command a lot of files are generated in the output folder. One of them is the assembly code derived from our logic in C.

Let's examine the addition.asm file. We ignore the various assembly assembly meta-commands and focus on the essentials for this tutorial:

.org 0x0000

__interrupt_vect:
	ljmp	__sdcc_gsinit_startup

__sdcc_program_startup:
	ljmp	_main

;	.\from-c-to-opcode\addition.c:5: void main(void)
;	-----------------------------------------
;	 function main
;	-----------------------------------------
_main:
;	.\from-c-to-opcode\addition.c:7: for (int i=0; i<3; i++){
	mov	r6,#0x00
	mov	r7,#0x00
00103$:
	clr	c
	mov	a,r6
	subb	a,#0x03
	mov	a,r7
	xrl	a,#0x80
	subb	a,#0x80
	jnc	00105$
;	.\from-c-to-opcode\addition.c:8: char x = P1;
;	.\from-c-to-opcode\addition.c:9: x = x - 0x34;
	mov	a,_P1
	add	a,#0xfb
	mov	_P1,a
;	.\from-c-to-opcode\addition.c:7: for (int i=0; i<3; i++){
	inc	r6
	cjne	r6,#0x00,00103$
	inc	r7
	sjmp	00103$
00105$:
;	.\from-c-to-opcode\addition.c:12: }
	ret

Let's examine the code step for step.

First we take a look on the code within the loop:

mov	a,_P1
add	a,#0xfb
mov	_P1,a

Instead of using the subb operation - we "misuse" the add operation and the fact how negative numbers are represented in most microcontrollers. Since Two's complement is used for representing negative numbers we can just rewrite to x = -5 + x or x = 0xFB + x.

Since those three operations presented above are located in code memory one after the other we can translate them into operation codes we find in the 8051 datasheet (p. 2-21ff):

mov	a,_P1   -> 0xE5 0x90
add	a,#0xfb -> 0x24 0xFB
mov	_P1,a   -> 0xF5 0x90

or in one line:

E59024FBF590

We find this string in the addition.ihx (line 4) Intel hex file which is flashed to the ROM.

The address of _P1 is 0x90.

Next, we address the for-loop. The assembly implementation of the foor loop consists of two aspects:

• incrementing i and exit the loop
• check if there is an overflow, since we use signed int for the iterator in the foor loop

Both aspects use their separate registers:

mov	r6,#0x00
mov	r7,#0x00

Register r6 is the proxy for i, r7 is used to check whether i is larger as 0x80 since it is the largest positive number for int. The destilled loop looks like following:

; initialize iterators for i and signed int check
mov	r6,#0x00
mov	r7,#0x00

00103$:
	clr	c

    ; carry bit will be zero if r6>=0x03
	mov	a,r6
	subb	a,#0x03

    ; carry bit will be zero if r7 > 128
	mov	a,r7
	xrl	a,#0x80
	subb a,#0x80

    ; quits, if carry bit is not set
	jnc	00105$

    ; ...
    ; ... Commands within the loop, described in the section above
    ; ...

    ; jump to 00103$
	inc	r6
	cjne	r6,#0x00,00103$

	inc	r7
	sjmp	00103$

00105$:
	ret

As you can see writing in C is much more pleasure than struggling with platform specific assembly code. Additionally SDCC provides various code optimization, e.g. converting multiplications into multiple additions.