A working machine vehicle , having - an articulated boom connected to the working machine vehicle, - a control system for controlling a moveable device which is the said articulated boom , the control system comprising: - at least one hydraulic actuator associated with the moveable device for performing a work function, wherein the moveable device has several positions incuding several basic positions and at least one extended position, and wherein reach (R) of the moveable device is dependent on the positions in such a way that when extended the reach (R) is longer than when in the basic positions, - at least one position sensing device to determine the position of the moveable device, and - a hydraulic control system , including an electronic control unit , for controlling the hydraulic actuator, wherein the hydraulic control system is configured to deliver pressurized hydraulic fluid to the hydraulic actuator, the hydraulic fluid being pressurized at most to a predetermined maximum pressure, characterized in that - the working machine vehicle is a forestry harvester or a forestry forwarder, - the work function is a load lifting work function, and - the control system is, while performing the load lifting work function, adapted to regulate the predetermined maximum pressure on the basis of the reach (R) in such a way that the predetermined maximum pressure is higher in the extended position than in the basic positions. - an articulated boom connected to the working machine vehicle, - a control system for controlling a moveable device which is the said articulated boom , the control system comprising: - at least one hydraulic actuator associated with the moveable device for performing a work function, wherein the moveable device has several positions incuding several basic positions and at least one extended position, and wherein reach (R) of the moveable device is dependent on the positions in such a way that when extended the reach (R) is longer than when in the basic positions, - at least one position sensing device to determine the position of the moveable device, and - a hydraulic control system , including an electronic control unit , for controlling the hydraulic actuator, wherein the hydraulic control system is configured to deliver pressurized hydraulic fluid to the hydraulic actuator, the hydraulic fluid being pressurized at most to a predetermined maximum pressure, - at least one hydraulic actuator associated with the moveable device for performing a work function, wherein the moveable device has several positions incuding several basic positions and at least one extended position, and wherein reach (R) of the moveable device is dependent on the positions in such a way that when extended the reach (R) is longer than when in the basic positions, - at least one position sensing device to determine the position of the moveable device, and - a hydraulic control system , including an electronic control unit , for controlling the hydraulic actuator, wherein the hydraulic control system is configured to deliver pressurized hydraulic fluid to the hydraulic actuator, the hydraulic fluid being pressurized at most to a predetermined maximum pressure, - the working machine vehicle is a forestry harvester or a forestry forwarder, - the work function is a load lifting work function, and - the control system is, while performing the load lifting work function, adapted to regulate the predetermined maximum pressure on the basis of the reach (R) in such a way that the predetermined maximum pressure is higher in the extended position than in the basic positions. <EOS>