In Raspberry Pi Model B Rev. 2,  there are four more GPIOs available on P5 headerless terminal as shown below that can be accessed if the 17 GPIOs on P1 expansion header are not enough for your application.

Since there is no header soldered on this terminal, you have the option to solder an 8 pin header or direct wire connection to utilize this extra GPIO port. However, soldering an 8 pin header on this port maybe a bit tricky and even dangerous for the inexperience because of lack of extra space and proximity to the P1 expansion header. I suggest to just solder a length of  #24 AWG wires on it and connect each wires to an insulated  'D' Connector female pins. 

To access and control these extra GPIOs under RISCOS, load first the GPIO Module and copy the following BBC BASIC Program code routines to Strong Editor and SAVE as BASIC then RUN.

THE PROGRAM CODE

 REM RASPBERRY PI MODEL B Revision 2 P5 Test

 REM Programmed by Rolly Estomaguio

 REM 10 August 2015

 MODE 20: OFF
 GCOL 3
 COLOUR 26: PRINT TAB(1,14); "Press Key to activate:"
 COLOUR 7: PRINT TAB(1,18); "H = All P5 GPIOs are HIGH"
 PRINT TAB(1,20); "L = All P5 GPIOs are LOW"
 COLOUR 18: PRINT TAB(1,1); "Raspberry Pi Model B Rev. 2 P5 GPIO Test"
 COLOUR 12: PRINT TAB(1,3); "PROGRAMMED BY: Rolly Estomaguio - 10 Aug. 2015"
 PROCinit
 PROC_p5_test
 

 DEFPROC_p5_test
 S%= INKEY (1)
 IF INKEY (-85) PROC_p5_high: REM "H" KEY
 IF INKEY (-87) PROC_p5_low: REM "L" key
 PROC_p5_test

 DEFPROCinit
 OSCLI "RMEnsure GPIO 0.00 RMLoad GPIO"
 OSCLI "RMEnsure GPIO 0.40 ERROR Please install the GPIO module"
 DIM G%(4)
 G%()= 28,29,30,31
 ENDPROC
 

 DEFPROC_p5_high
 SYS "GPIO_WriteMode",28,1: SYS "GPIO_WriteData",28,1
 SYS "GPIO_WriteMode",29,1: SYS "GPIO_WriteData",29,1
 SYS "GPIO_WriteMode",30,1: SYS "GPIO_WriteData",30,1
 SYS "GPIO_WriteMode",31,1: SYS "GPIO_WriteData",31,1
 FOR D%= 1 TO 50000: NEXT
 H%= INKEY (1)
 IF INKEY (-99) PROC_p5_low
 IF INKEY (-87) PROC_p5_low
 PROC_p5_high
 

 DEFPROC_p5_low
 SYS "GPIO_WriteMode",28,1: SYS "GPIO_WriteData",28,0
 SYS "GPIO_WriteMode",29,1: SYS "GPIO_WriteData",29,0
 SYS "GPIO_WriteMode",30,1: SYS "GPIO_WriteData",30,0
 SYS "GPIO_WriteMode",31,1: SYS "GPIO_WriteData",31,0
 FOR D%= 1 TO 50000: NEXT
 L%= INKEY (1)
 IF INKEY (-99) PROC_p5_high
 IF INKEY (-85) PROC_p5_high
 PROC_p5_low

OR YOU CAN USE:

DEFPROC_p5_high

FOR P%= 28 TO 31

SYS "GPIO_WriteMode",P,1: SYS "GPIO_WriteData",P%,1

NEXT

L%= INKEY (1)

IF INKEY (-87) PROC_p5_low

PROC_p5_high

DEFPROC_p5_low

FOR P%= 28 TO 31

SYS "GPIO_WriteMode",P,1: SYS "GPIO_WriteData",P%,0

NEXT

L%= INKEY (1)

IF INKEY (-85) PROC_p5_high

PROC_p5_low