When calling the OSWORD routine from machine code you will need to use the absolute address &40F1. However, when CALLing this routine from BBC BASIC it is recommended that you use the BBC Micro-compatible address &FFF1.


The OSWORD routines operate in a similar manner to the OSBYTE routines except that instead of parameters being passed in via registers they are stored in a parameter block in RAM, the address of which is passed via the HL register pair.


The routine number is loaded into the Z80's accumulator register A. When CALLing the routine from BBC BASIC you will need to set A% to the routine number.

Parameters for the routine are stored in RAM. You will need to set the Z80 register pair HL to the address of this parameter block, which in BBC BASIC can be accomplished via the H% and L% static variables.

OSWORD &01 - Read system clock

This routine reads the relative system clock (used for the BASIC TIME function). It outputs a five-byte integer to the address in HL.

20 DIM time 4
30 H%=time/256:L%=time
40 A%=&01
60 PRINT !time

OSWORD &02 - Write system clock

This routine sets the system clock to the five-byte integer value pointed to by HL.

OSWORD &09 - Read pixel value

This routine returns the colour of a pixel at a specified point on the screen in the same manner as thePOINT function.

The inputs are as follows:

On exit,

OSWORD &0D - Read last two graphics cursor positions

The host interface keeps track of the last two visited graphics cursor positions (triangle and parallelogram filling require three points; two remembered and one passed with the PLOT command). You can use this routine to retrieve these recorded positions. HL needs to point to eight bytes of memory to receive the following values:

The values returned are all absolute coordinates in relation to the top-left hand corner of the screen.

An example of the usage of this routine is shown below. It is an assembly program that can be used to retrieve the current graphics origin (set with VDU 29 command). It works by moving the graphics cursor to (0,0) then reading back the current cursor position.

 10 REM OS call routine constants
 20 OSWRCH=&40EE
 30 OSWORD=&40F1
 40 :
 50 DIM GetOrigin 90
 60 FOR pass%=0 TO 1
 70   P%=GetOrigin
 80   [
 90   OPT pass%*2
100   ;
110   ; Check there are 2 arguments
120   LD A,(IX+0) : CP 2 : RET NZ
130   ;
140   ; Check they are both integers
150   LD A,(IX+1) : CP 4 : RET NZ
160   LD A,(IX+4) : CP 4 : RET NZ
170   ;
180   ; MOVE to the origin by invoking VDU 25,5,0;0;
190   LD A,25 : CALL OSWRCH
200   LD A,5 : CALL OSWRCH
210   ; Output 4 zeroes to move to (0,0).
220   LD B,4
230   .lp XOR A : CALL OSWRCH : DJNZ lp
240   ;
250   ; Invoke OSWORD &0D to read last two cursor positions
260   LD HL,PrevX
270   LD A,&0D : CALL OSWORD
280   ;
290   ; Now we know the origin we need to output it.
300   LD L,(IX+2) : LD H,(IX+3)
310   LD DE,(CurrX) : CALL OutputCoord
320   LD L,(IX+5) : LD H,(IX+6)
330   LD DE,(CurrY) : CALL OutputCoord
340   RET
350   ;
360   ; 8 bytes of storage for cursor positions
370   .PrevX : DEFW 0 : .PrevY : DEFW 0
380   .CurrX : DEFW 0 : .CurrY : DEFW 0
390   ;
400   ; Outputs 16-bit value DE to 32-bit variable at HL
410   .OutputCoord
420   LD (HL),E : INC HL
430   LD (HL),D : INC HL
440   SLA D
450   SBC A,A
460   LD (HL),A : INC HL
470   LD (HL),A : RET
480   ]
490 NEXT pass%

The routine can now be CALLed to retrieve the current graphics origin:

VDU 29,48;-32;
CALL GetOrigin,x%,y%
PRINT "Origin = (";x%;",";y%;")"

See Also