USR

USR is a BASIC function to call (execute) a machine code routine in memory or BBC MOS API entry. It returns a numerical result back to BASIC. On most non-6502 platforms, BBC BASIC intercepts calls to the standard BBC MOS API entries and translates them to an equivalent action.

USR

Availability	Present in all versions of BBC BASIC.
Syntax	BASIC I-V	<num-var> = USR(<numeric>)
Token (hex)	BASIC I-V	BA (function)
Description	BASIC I-V	Sets the main CPU registers to the values of the resident integer variables, and calls a machine code routine at the address given in the <numeric>. The routine should exit with the normal subroutine return instruction. After the routine exits, USR returns an integer holding values returned in CPU registers.
Associated keywords	?, !, $, CALL

Description

USR allows a BASIC program to use a piece of machine code as part of its operation. Machine code is a program in the form of binary data that can be run directly by the central processing unit (CPU).

USR is one of the ways to access functions of the MOS (its Application Programming Interface, or API.) Advanced programmers can also use USR to improve the performance of a BASIC program by rewriting the most time-consuming parts in machine code, as it runs much, much faster than the equivalent BASIC code.

Because USR is involved with the low level architecture of the computer, its precise function depends on which CPU is running BASIC. The CPU-specific preparation is the same as in CALL, except that there is no table of variables as USR's only argument is the <numeric> address. In general, USR sets certain CPU registers to the values of the resident integer variables, and jumps into a machine code routine at the address given in the <numeric>.

After the routine exits, BASIC captures a certain portion of the CPU's state, and returns it as the numeric result of the USR function. The value that is returned is again CPU-dependent:

BBC MOS API Access

To ensure cross-platform compatibility, most non-6502 implementations of BBC BASIC translate CALLs and USR to MOS API entries to an equivalent call to the underlying MOS. The only addresses that are supported by this are those listed below. The values in A%, X% and Y%, or the data or control block pointed to by X%, are passed in an appropriate manner to the underlying system.

Address	MOS call	Action
&FFF7	OSCLI	Execute *command.
&FFF4	OSBYTE	Various byte-wide functions.
&FFF1	OSWORD	Various control block functions.
&FFEE	OSWRCH	Write character to output stream.
&FFE7	OSNEWL	Write NewLine to output stream.
&FFE3	OSASCI	Write character or NewLine to output stream.
&FFE0	OSRDCH	Wait for character from input stream.
&FFDD	OSFILE	Perform actions on whole files or directories.
&FFDA	OSARGS	Read and write information on open files or filing systems.
&FFD7	OSBGET	Read a byte from an a channel.
&FFD4	OSBPUT	Write a byte to a channel.
&FFD1	OSGBPB	Read and write blocks of data.
&FFCE	OSFIND	Open or close a file.

Register usage

CPU	Registers on entry	Result returned by USR
6502	A=A%, X=X%, Y=Y%, Cy=b0 of C%	b0-7=A, b8-15=X b16-23=Y, b24-31=P
6809	A=A%, B=B%, U=U%, X=X%, Y=Y%, Cy=b0 of C%	b0-7=A, b8-15=X b16-23=Y, b24-31=CC
Z80	A=A%, B=B%, C=C%, D=D%, E=E%, H=H%, L=L%	b0-b15=HL' b16-b31=HL
Z80 BBC API call	A=A%, L=X%, H=H%, E=E%	b0-b7=A, b8-b23=HL, b24-31=F
32016	Testing and disassembly shows 32016 BASIC only sets up registers for CALL parameter block or for MOS API calls	b0-b31=R1
32016 BBC API call	R1=A%, R2=X%, R3..R7=contents of control block at X%	b0-b7=R1, b8-b23=R2
PDP11	R0=A%, R1=B%, R2=C%, R3=D%, R4=E%, R5=F%	b0-b15=R0, b16-b31=R1
PDP11 BBC API call	R0=A%, R1=X%, R2=Y%	b0-b7=R0, b8-b23=R1
ARM	R0=A%, R1=B%, R2=C%, R3=D%, R4=E%, R5=F%, R6=G%, R7=H%, R8=I%, R9=J%	b0-b31=R0
ARM BBC API call	R0=A%, R1=X%, R2=Y%, or R1..R5=contents of control block at X%	b0-b7=R0, b8-b31=R1

When passing the address of a data structure or control block, code similar to the following should be used.

   DIM ctrl% 31      :REM Control block, may be anywhere in 32-bit memory space
   X% = ctrl%        :REM X% holds full 32-bit address
   Y% = X% DIV 256   :REM Y% holds 32-bit address, shifted right 8 bits

On 8-bit platforms the high 24 bits of X% and Y% are ignored, and the control block is found with X%+256*Y%. On platforms with larger registers, the control block is found at X%, ignoring Y% completely. Some platforms will check if X%<256 and use X%+256*Y%, though this should not be relied on.

An additional advantage is that the control block is easily accessed with X%, for example, X%!2=load%, etc.

Notes

Yes, 32000 passes A%=R1, etc., not R0.

When calling &FFCE with A%=0 to close a file, 6502 BASIC requires the channel to be passed in Y% but in ARM BASIC the channel must be passed in X%. When calling from BASIC, the CLOSE# statement should be used.

-- beardo 19:06, 5 May 2007 (BST) Jgharston 17:40, 8 October 2007 (BST) Jgharston (talk) 15:02, 19 August 2017 (CEST)