Thursday, July 4, 2013

Sega BASIC Programming

In the previous post, we checked out Sega Retro Gaming and some classic cartridge-based video games built on early 8-bit hardware such as the Sega SG-1000, the SC-3000 and the Sega Master System.

Now, we would like to write our own similar 8-bit retro games!

In this post, we will focus on the Sega SC-3000 home computer using the BASIC programming language for its simplicity and ease-of-use.

In order to write Sega BASIC code on a modern-day computer, you will need an emulator installed and a copy of the Sega BASIC Level 3 ROM, or Sega BASIC IIIb, as it's also known.

While there are many emulators available for Sega platforms, only one emulator seems to be compatible with the Sega BASIC Level 3 ROM, that is: Meka
  • Download mekaw073.zip and extract
  • Double click on mekaw.exe to launch
Note: check Debugger: Enable Z80 debugger for better developer experience.

Configuration
Meka may be challenging to work with, especially if you have dual monitors. In order to alternate windowed / full screen mode, make changes to the meka.blt file, similar to the following:
[HQ2X::WIN]
res = 1024x768
driver = auto_win
refresh_rate = 20
blitter = hq2x
stretch

  • Launch Meka
  • Video menu
  • Blitters
  • Select HQ2X
Note: press ESC to alternate windowed / full-screen mode.

Sega BASIC
Next, download the Sega BASIC Level 3 ROM:
26620 Bytes free – are you serious?!?

Setup
Each time Meka is launched, activate the Sega keyboard to begin typing:
  • Inputs menu
  • Sega Keyboard
Note: the Sega keyboard is not 100% compatible with typical QWERTY keyboard.
For example the "&" character is Shift+6 not Shift+7 on the Sega keyboard.

Hello World
Type in the first BASIC program: the obligatory "Hello World":
NEW
10 PRINT "HELLO WORLD"
20 GOTO 10
RUN
Hello World will be printed down the screen indefinitely. Press the "End" key to break.
Note: there can only be one active program in memory at any one time.

Shortcuts
Here is short list of keyboard shortcuts that may be useful when using the Sega BASIC Level 3 ROM:

Key
Scroll lock
End key
Arrow keys
Backspace
Shift + Backspace

Action
soft reset (keeps code in memory)
break current program execution
move cursor around text screen
delete character to left of cursor
insert character to right of cursor

Also, the Sega keyboard has a special function key that simplifies the typing of entire BASIC commands, for example, press of the "FUNC" key and one other key combination:

Command
LIST
RUN

Shortcut
Tab + Backspace
Tab + ` (backtick)
Note: shortcuts may be different depending on localization.

State
After typing in the first BASIC program, it will remain in memory until the Sega computer is powered off. In order to maintain program state across sessions, simply save state, close and load state next session.
  • Main menu
  • Save State
  • Close Meka
  • Launch Meka
  • Main menu
  • Load State
Note: file is stored at C:\PathToMeka\Saves\Sega BASIC Level 3 V1 (SC-3000) [b1].S00

Important: there can only be one copy of the state file at any one time.
Therefore, it is up to you to archive your state files accordingly!

Documentation
The Sega SC-3000 Survivors web site contains links to much documentation: manuals, magazines and books. SMS Power web site also has more magazines scans for Sega 8-bit preservation and fanaticism!

Example
As an example, let's write the main loop of a simple "Shoot 'em up" style game that simply moves a target sprite around the screen based on user input; preferably the joystick.

Controller
If you have a joystick connected to your computer, e.g. wireless Xbox 360 controller;
Configure the controller for best game play experience:
  1. Close Meka
  2. Activate controller
  3. Launch Meka
  4. Inputs menu
  5. Joypad checked
  1. Configuration
  2. Input Configuration
  3. Click ">>" twice
  4. 3/6 Joypad 1
  5. Enabled checked
Type in the following BASIC program to confirm the controller is configured correctly:
NEW
10 PRINT STICK(1)
20 GOTO 10
RUN
Zeros will be printed down the screen indefinitely: move the joystick in different directions;
Output: a value between 1 to 8 will be printed on-screen. Press the "End" key to break.

Sample
The following code sample is reminiscent of the Sega BASIC "Shoot 'em up" game titled "3D City" (1988). While we will not dissect the full program at this time, we will replicate the main loop accordingly.

Type in the following program exactly as is:
10 REM SEGA BASIC PROGRAMMING
20 GOSUB 300
30 ST=1
40 SZ=16:MAG1
50 ML=0:MU=0
60 MR=256-SZ:MD=192-SZ
70 TX=MR/2:TY=MD/2
80 SCREEN 2,2:CLS
90 COLOR,1,(0,0)-(255,191),1
100 REM MAIN LOOP
110 SPRITE 0,(TX,TY),0,9
120 VX=0:VY=0
130 S1=STICK(1):FR=STRIG(1):A$=INKEY$
140 IF S1=3 OR A$=CHR$(28) THEN VX=1:GOSUB 200
150 IF S1=7 OR A$=CHR$(29) THEN VX=-1:GOSUB 200
160 IF S1=1 OR A$=CHR$(30) THEN VY=-1:GOSUB 200
170 IF S1=5 OR A$=CHR$(31) THEN VY=1:GOSUB 200
180 IF FR=1 OR A$=CHR$(32) THEN END
190 GOTO 100
200 REM MOVEMENT
210 VX=VX*ST:VY=VY*ST
220 TX=TX+VX:TY=TY+VY
230 IF TX <= ML THEN TX=ML
240 IF TX >= MR THEN TX=MR
250 IF TY <= MU THEN TY=MU
260 IF TY >= MD THEN TY=MD
270 RETURN
300 RESTORE 350
310 FOR S=0 TO 3:READ S$:PATTERN S#S,S$:NEXT S
320 RETURN
350 DATA 000039202100012A, 2A01002120390000
360 DATA 00009C0484008054, 54800084049C0000
RUN
The graphics screen clears black and positions a red target sprite. Use the controller, or arrow keys, to move the target sprite left, right, up and down. Press the left fire button or space bar to end program. As you can see, the current per-pixel sprite movement is smooth but extremely slow!
Therefore, increase the movement step; change line 30 of the code to the following:
30 ST=2        30 ST=4        30 ST=8        30 ST=16        etc.
RUN

Also, the movement step can be changed dynamically at runtime by manipulating memory directly:
  • Tools menu
  • Memory Editor
  • Click RAM button
  • Type EFF0
  • Enter "01"
Note: &HEFF0 is the 16-bit memory address where ST will be manipulated.
Next, add the following lines of code to the original program:
34 AD=&HEFF0
38 POKE AD,ST
205 ST=PEEK(AD)
RUN
Program runs as before. However, this time experiment: enter hexadecimal values at the Memory Editor address &HEFF0 for example: 02, 04, 08, 10 etc. to see the movement step effects on the target sprite.

Finally, save the state of Meka for future use: Main menu, Save State.

Summary
In this post, BASIC programming was introduced on the Sega SC-3000 for simplicity and ease-of-use.

However, games written in BASIC suffer from the disadvantage that they are too slow for applications such as sprite movement; these may be sped up by increasing the step but precision of control is lost.

This can be resolved by using machine code routines for parts of the program where BASIC is too slow. This will be the topic in the next post.

2 comments:

mahasiswa teladan said...

hi..Im student from Informatics engineering, this article is very informative, thanks for sharing :)

SteveProXNA said...

Hi Mahasiswa,

Your welcome. Glad you enjoyed the article.

Cheers,
Steve.

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