Hello Jan,
thanks for help.

I use Monitor-51 V2.1 - this is all I know now.

Basic HW components:

1. Processor TEMIC TSC80C31
2. EPROM 27C64 mapped at C:0000 - C:1FFF, mirror at C:2000 - C3FFF; contains monitor
3. RAM - von Neumann: C+X:A000 - BFFF (BF00 - BFFF is dedicated for MON51 data)
4.MAX232 for the serial line

My code:

#pragma iv(0xA000)
#include "reg51.h"
#include <stdio.h> /* prototype declarations for I/O functions */
#define zakl_perioda 0xFFFF - 10000 //10,8507 ms
#define krok 2000

#ifdef MONITOR51 /* Debugging with Monitor-51 needs */
char code reserve [3] _at_ 0xA023; /* space for serial interrupt if */
#endif /* Stop Exection with Serial Intr. */
/* is enabled */


unsigned int Pocet_aktivnich; //inkrementovat/dekrementovat po stovkach
sbit motor = P1^7;
void main(void) {
TMOD = 0x11; //nastveni t0 a t1 coby 16bit casovacu s prednastavovanim
IP = 0x0A; //timery mají prioritu pøed externím pøerušením
IE = 0x8F;

TL1 = (zakl_perioda) & 0xFF;
TH1 = (zakl_perioda >> 8) & 0xFF;
//TH1 = 0xD8;
IT0 = 1;
IT1 = 1;
Pocet_aktivnich = (0xFFFF-(80*100));

//printf ("Hello World\n");

TR1 = 1;

while (1)
;
}
void int_tim1 (void) interrupt 3 {
motor = 1;
TL1 = (zakl_perioda) & 0xFF;
TH1 = (zakl_perioda >> 8) & 0xFF;

TL0 = (Pocet_aktivnich) & 0xFF;
TH0 = (Pocet_aktivnich >> 8) & 0xFF;

TR0 = 1;
}
void int_tim0 (void) interrupt 1 {
motor = 0;
TR0 = 0;
}
void int_ext0 (void) interrupt 0 {
Pocet_aktivnich -= krok;
}
void int_ext1 (void) interrupt 2 {
Pocet_aktivnich += krok;
}