MODE
MODE is a BASIC statement to change the display mode. A display mode is a particular set-up of the video display, with regards to its width, height, resolution, the number of colours available and the video standard (such as PAL, NTSC or VGA). There is usually a limited number of display modes available on any given machine.
| Availability | Present in all original versions of BBC BASIC. | |
| Syntax | BASIC I-IV | MODE <numeric>
|
| BASIC V,VI | MODE <numeric>|<string>
| |
| BASIC V and some other BASICs |
<numeric> = MODE
| |
| Token (hex) | All BASICs | EB (statement, rvalue)
|
| Description | All BASICs | Validates the new mode against available memory, sets the display mode, and sets HIMEM to the new end of BASIC memory where appropriate.
|
| As a function | Returns the number of the current display mode. | |
| Associated keywords | CLS, CLG, HIMEM
| |
Description
MODE sets the display mode to the number or string given as the
argument. In some BASICs it can also be used as a function to return the
the current mode number.
What the number or string means is dependent on the underlying OS; BASIC is not too concerned with the details, except that 6502 BASIC, when running on the I/O processor, anticipates that the amount of memory available to it will change.
6502 BASIC first finds out what processor it is running on using OSBYTE &82.
If it is running on the I/O processor, it then finds what the start of
display memory would be in the new mode using OSBYTE &85. If the display
area would overlap the heap (whose end address is VARTOP), it aborts
with a Bad MODE error. To free enough memory for the mode
change, the user may need to discard all variables with the
CLEAR statement, or reduce the size of the program with
editing, DELETE or even NEW.
6502 BASIC also gives a Bad MODE error if the MODE
statement is in a procedure or function when running on the I/O processor --
meaning the BASIC stack is in use, which would definitely be lost if
HIMEM were to change.
If there is no memory conflict, BASIC issues a VDU 22
command with the new mode as a parameter, and sets HIMEM to the
new display address. Non-6502 BASICs simply go ahead with issuing the
VDU 22 sequence, as by definition, they cannot be running in
the 6502 I/O memory.
BASIC has no way of knowing when the display area has changed in size; if
a VDU 22 command is issued other than by MODE,
HIMEM will not be updated and part of the user's memory may be
wiped, with potentially disastrous results.
Under RISC OS, BASIC calls SWI &65 "OS_ScreenMode" either to set the
numerical mode or return its number to the user. If the mode to be set is a
string, it issues the command *WimpMode <string>. The display
memory does not directly interact with BASIC's application memory area, but
external influences on this area are handled by a separate mechanism.
OSBYTE &87 reads the current screen MODE number, but without the shadow screen bit if a shadow screen is selected. Reading screen mode shows how to determine if a shadow screen is selected.
Display modes
| MODE | Type | Resolution | Colours | Characters | Pixel aspect | Bits per pixel |
Memory | Address range | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Graphics | Text | Size | Definition | |||||||
| 0 | ASCII | 640 × 256 | 80 × 32 | 2 | 8 × 8 | User | 1:2 (tall) | 1 | 20K | &3000..&7FFF |
| 1 | ASCII | 320 × 256 | 40 × 32 | 4 | 8 × 8 | User | 1:1 (square) | 2 | 20K | &3000..&7FFF |
| 2 | ASCII | 160 × 256 | 20 × 32 | 16 | 8 × 8 | User | 2:1 (wide) | 4 | 20K | &3000..&7FFF |
| 3 | ASCII | -- | 80 × 25 | 2 | 8 × 10 | User | 1:2 (tall) | 1 | 16K | &4000..&7FFF |
| 4 | ASCII | 320 × 256 | 40 × 32 | 2 | 8 × 8 | User | 1:1 (square) | 1 | 10K | &5800..&7FFF |
| 5 | ASCII | 160 × 256 | 20 × 32 | 4 | 8 × 8 | User | 2:1 (wide) | 2 | 10K | &5800..&7FFF |
| 6 | ASCII | -- | 40 × 25 | 2 | 8 × 10 | User | 1:1 (square) | 1 | 8K | &6000..&7FFF |
| 7 | Teletext | 78 × 75* | 40 × 25 | 8** | 12 × 20 | Hardware | 1:1 (square) | 1 | 1K | &7C00..&7FFF |
| 8 | ASCII (Archimedes/RISC OS) | 640 × 256 | 80 × 32 | 4 | 8 × 8 | User | 1:2 (tall) | 2 | 40K | |
| 8 | Unofficial 4-colour MODE 0/condensed MODE 1 |
320 × 256 | 80 × 32 | 4 | 4 × 8 | User | 1:1 (square) | 2 | 20K | &3000..&7FFF |
| 8 | Unofficial lower-res MODE 2 |
80 × 256 | 10 × 32 | 16 | 8 × 8 | User | 4:1 (ultra wide) | 4 | 10K | &5800..&7FFF |
| 9 | ASCII (Archimedes/RISC OS) | 320 × 256 | 40 × 32 | 16 | 8 × 8 | User | 1:1 (square) | 4 | 40K | |
| 9 | Unofficial condensed MODE 2 |
160 × 256 | 40 × 32 | 16 | 4 × 8 | User | 2:1 (wide) | 4 | 20K | &3000..&7FFF |
| 9 | Unofficial lower-res, text-only MODE 2 |
-- | 10 × 25 | 16 | 8 × 10 | User | 4:1 (ultra wide) | 4 | 8K | &6000..&7FFF |
| 10 | ASCII (Archimedes/RISC OS) | 160 × 256 | 20 × 32 | 256 | 8 × 8 | User | 2:1 (wide) | 8 | 40K | |
| 10 | Unofficial text-only MODE 5 |
-- | 20 × 25 | 4 | 8 × 10 | User | 2:1 (wide) | 2 | 8K | &6000..&7FFF |
| 11 | ASCII (Archimedes/RISC OS) | -- | 80 × 25 | 4 | 8 × 10 | User | 1:2 (tall) | 2 | 40K | |
| 11 | Unofficial 4-colour MODE 3/condensed MODE 3 |
-- | 80 × 25 | 4 | 4 × 10 | User | 1:1 (square) | 2 | 20K | &3000..&7FFF |
| 12 | Unofficial 4-colour MODE 4/condensed MODE 5 |
160 × 256 | 40 × 32 | 4 | 4 × 8 | User | 2:1 (wide) | 2 | 10K | &5800..&7FFF |
| 13 | Unofficial 16-colour MODE 5/condensed MODE 5 |
80 × 256 | 20 × 32 | 16 | 4 × 8 | User | 4:1 (ultra wide) | 4 | 10K | &5800..&7FFF |
| 14 | Unofficial 4-colour MODE 6/condensed MODE 6 |
-- | 40 × 25 | 16 | 4 × 8 | User | 2:1 (wide) | 4 | 8K | &5800..&7FFF |
| &60 | 1-bit extended-colour MODE 0 |
-- | 80 × 32 | 5 | 7 × 8 | User | 1:2 (tall) | 1 | 20K | &3000..&7FFF |
| &61 | 1-bit extended-colour MODE 1 |
-- | 40 × 32 | 13 | 7 × 8 | User | 1:1 (square) | 2 | 20K | &3000..&7FFF |
| &63 | 1-bit extended-colour MODE 3 |
-- | 80 × 25 | 5 | 7 × 8 | User | 1:2 (tall) | 1 | 16K | &4000..&7FFF |
| &64 | 1-bit extended-colour MODE 4 |
-- | 40 × 32 | 5 | 7 × 8 | User | 2:2 (square) | 2 | 16K | &4000..&7FFF |
| &66 | 1-bit extended-colour MODE 6 |
-- | 40 × 25 | 5 | 7 × 8 | User | 1:1 (square) | 2 | 10K | &5800..&7FFF |
| &68 | 2-bit extended-colour MODE 0 |
-- | 80 × 32 | 9 | 6 × 8 | User | 1:2 (tall) | 1 | 20K | &3000..&7FFF |
| &69 | 2-bit extended-colour MODE 1 |
-- | 40 × 32 | 16 | 6 × 8 | User | 1:1 (square) | 2 | 20K | &3000..&7FFF |
| &6B | 2-bit extended-colour MODE 3 |
-- | 80 × 25 | 9 | 6 × 8 | User | 1:2 (tall) | 1 | 16K | &4000..&7FFF |
| &6C | 2-bit extended-colour MODE 4 |
-- | 40 × 32 | 9 | 6 × 8 | User | 2:2 (square) | 2 | 16K | &4000..&7FFF |
| &6E | 2-bit extended-colour MODE 6 |
-- | 40 × 25 | 9 | 6 × 8 | User | 1:1 (square) | 2 | 10K | &5800..&7FFF |
(*) User-generated graphics. (**) Colours via Teletext control codes.
The Model A supports modes 4 to 7; the Electron, modes 0 to 6; and all other BBC Micros, modes 0 to 7.
The B+ and Master series can set up a shadow screen
that resides in shadow RAM and leaves the top of main memory free for
the user. Shadow mode is selected by adding 128 to the mode number (hence
MODEs 128 to 135), or giving the command *SHADOW before a mode
change. As a result HIMEM is always reset to &8000, the end of
main memory.
The Archimedes does not have shadow RAM, but supports many more modes.
| Screen display MODEs |
|---|
|
MODE 0 | MODE 1 | MODE 2 | MODE 3 | MODE 4 | MODE 5 | MODE 6 | MODE 7 |
beardo 16:24, 14 September 2007 (BST)
