ARM programming for Risc OS

This page is Under Construction.... 'Learn ARM is planned for 2020'

RISC OS was the operating system of the Acorn Archimedes... although the machine is long since dead, RISC OS has survived and ported to modern machines like the Raspberry Pi

In these tutorials we'll be testing with Risc OS 3.57 - which is the OS of the successor of the Archimedes, but where possible we'll try and support the classic RiscOS 2... and we'll be using instructions compatible with ARM2 for backwards compatibility!

These tutorials use the VASM assembler

Acorn Archimedes versions

	A3000	A5000	RISC PC
CPU	8mhz ARM2	25 mhz ARM3	30mhz ARM 610
Memory	1 MB	1-8 MB	?
OS	RISC OS 2	RISC OS 3	Risc OS 3.5 - 4.03

If you want to learn ARM get the Cheatsheet! it has all the ARM7 commands, it covers the commands, and options like Bitshifts and conditions as well as the bytecode structure of the commands!

Resources

RPCEMU - Risc OS emulator
Rom Images - Images for RISC OS

Executable files

The easiest way to create a program is save it as a FF8.... eg 'prog,ff8'... NOTE: this is a COMMA before ff8 not a full stop - this is how RISC OS denotes file type

Our program just needs to start from 0x8000 and it will work fine!... please note, you'll need to create your own stack area and define a stack pointer if you need the Stack, as the OS doesn't create one!

The Example Hello World to the right will compile correctly with VASM

.org 0x08000
    SWI 01                    ;OS_WriteS
    .ascii "Hello World!"
    .byte 0                   ;End of string
    .align 4

     MOV R0,#0                ;0=No Error
     SWI 0x11               ;OS_Exit

We can also create a !Run,feb to autostart the program with the emulator, we

RMEnsure UtilityModule 3.10 Error This !Boot structure is only suitable for RISC OS 3.1 or later
Set Boot$Dir <Obey$Dir>
Set Boot$Path <Boot$Dir>.
desktop prog

Using Risc OS

When using Risc OS, you will often want the options on a file... to get this use the MIDDLE MOUSE button
You can get a command prompt by pressing CTRL-F12, or selecting Task Window by middle clicking on the Acorn icon The commands you can use are known as Star Commands... there is a list of them here

VDU Commands
Many screen functions are performed via 'VDU Commands'... to use these, we just write special character codes to the screen with SWI 0

Page	Meaning	Parameters	Example
VDU 0	Does nothing	0
VDU 1	Sends next character to printer only	1,c
VDU 2	Enables printer	2
VDU 3	Disables printer	3
VDU 4	Writes text at text cursor	4
VDU 5	Writes text at graphics cursor	5
VDU 6	Enables VDU driver	6
VDU 7	Generates bell sound	7
VDU 8	Moves cursor back one character	8
VDU 9	Moves cursor on one space	9
VDU 10	Moves cursor down one line	10
VDU 11	Moves cursor up one line	11
VDU 12	Clears text window	12
VDU 13	Moves cursor to start of current line	13
VDU 14	Turns on paged mode	14
VDU 15	Turns off paged mode	15
VDU 16	Clears graphics window	16
VDU 17	Defines text colour	17,color
VDU 18	Defines graphics colour	18,maskmode,color
VDU 19	Defines logical colour	19,color,mode,r,g,b	19,0,16,255,128,0
VDU 20	Restores default logical colours	20
VDU 21	Disables VDU drivers	21
VDU 22	Selects screen mode	22,ScreenMode	22,9
VDU 23	Multi-purpose command:	n,n,n,n,n,n,n,n,n
VDU 23	Sets the interlace and controls cursor appearance	?
VDU 23	Controls text cursor appearance	?
VDU 23	Defines ECF pattern and colours	?
VDU 23	Sets dot-dash line style	?
VDU 23	Scrolls text window or screen	?
VDU 23	Clears a block of the text window	?
VDU 23	Sets first flash time	?
VDU 23	Sets second flash time	?
VDU 23	Sets default patterns	?
VDU 23	Defines simple ECF patterns and colours	?
VDU 23	Controls cursor movement after printing	?
VDU 23	Sets the tint for a colour	?
VDU 23	Chooses ECF patterns	?
VDU 23	Exchanges text foreground and background colours	?
VDU 23	Sets ECF origin	?
VDU 23	Sets character size/spacing	?
VDU 23	Reserved for future expansion	?
VDU 23	Private Font Manager calls	?
VDU 23	Private Sprite Manager calls	?
VDU 23	Reserved for use by application programs	?
VDU 23	Redefines printable characters	?
VDU 24	Defines graphics window	24,x1,y1,x2,y2
VDU 25	PLOT command	25,x,y
VDU 26	Restores default windows	26
VDU 27	Does nothing	27
VDU 28	Defines text window	28,x1,y1,x2,y2
VDU 29	Defines graphics origin	29,x,y
VDU 30	Homes text cursor	30
VDU 31	Moves text cursor	31,x,y
VDU 127	Delete	127

SWI commands
SWI's are systems calls to the operating system... many SWI's were added in later operating system versions, so ensure you only use SWI's that exist on the version you want to support.