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JBit

Tutorial

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VM

JBit is a complete programming environment and is more complex then a typical mobile phone application. This tutorial will help you to get started using JBit.

The first thing to understand is that, when you are writing a program using JBit, you are not writing a program for your mobile phone, but for the JBit Virtual Machine (MicroIO Edition), or VM.

Here there is a picture of the VM, and you can see that it looks like an ugly mobile phone. Now, the VM might be ugly, but it is definitely not a mobile phone. For a start, you cannot make or receive calls, nor send or receive text messages with it. Furthermore, the display can only show a matrix of 10x4 characters and the keypad has only the standard keys (i.e.: 0-9, * and #).

Since the VM cannot function as a phone, what can it be useful for? Well, the VM is pretty much useless. However, learning how to program it can help you to understand how computers work. Computers are complex devices and their inner core is hidden by several layers of abstraction, but they do have a core and their core is not much more complex than the VM.

Main Menu (Editor)

Another thing to understand is the difference between the VM and JBit. JBit includes the VM, but it is not only that. It also includes tools to help you to edit and manage programs for the VM.

Actually, you cannot reach the VM until you make a program for it. The VM itself is empty and has no such a thing such a Desktop that you can play with. Therefore, our first task will be to create an minimal program, so that we can have a look at the VM.

Select Editor from the list.

New

The Editor needs to know how big the program is. At this stage we do not really care and so we can just confirm 4 Code Pages and 3 Data Pages.

Select OK.

On some phones, OK is directly mapped to a soft key, while on other phones, OK is available using a menu (e.g. Options or Menu). If you do not see the Size (New) screen, it probably means that you already have one program loaded and should restart JBit.

Empty Prog.

You should now see a matrix of zeros. This is the program. We will come back to it later. For now, we only want to quickly get to the VM.

Select Debug.

CPU (1)

Here is the VM. As you can see, it does not look anything like the picture above. This is because you are actually looking inside the VM!

Press # to switch to the outside view.

Display (Empty)

Now you are looking at the display of the VM. There is no point in showing you the keypad of the VM, as the keypad of your phone can act as the keypad of the VM.

There is a minor complication here. If we started the VM properly, the VM would have halted immediately, because the program is empty. Instead, we started the VM in a frozen mode that allows us to inspect it. In this mode of operation, it is far more common to look at the display than to use the keypad, so, right now, the keypad of your phone is not acting as the keypad of the VM. Instead, the keypad of your phone is used to switch back to the inside view.

Press # again to switch back to the inside view.

CPU (BRK)

Let us have a look at what is inside the VM.

You have probably already heard of the term CPU (Central Process Unit), as it is one of the criteria of selection when shopping for a computer. The CPU used by the VM is essentially a 6502, a CPU popular during the 70s and the 80s. It is smaller and slower, but, from a theoretical point of view, no less powerful than a modern CPU.

Gaining an understanding of the CPU is the whole point of JBit and will take a while. We can start with this: the CPU is an agent acting on a virtual world on our behalf. Before letting the CPU act on our behalf, we will play a bit with this virtual world ourselves.

Press 0 to switch to the MEMory view.

Memory (Empty)

Unlike in the real world, where we are free to place objects as we like, in this virtual world, objects are placed in cells and cells are disposed in a single row.

Also, unlike in the real world, where there is a variety of objects, in this virtual world, there is only one kind of object: the byte, a small integer ranging from 0 to 255.

Finally, unlike the real world, this virtual world is finite. There are exactly 65536 cells. To better manage this long sequence of cells , we partition it in 256 segments (called pages) of 256 cells each. This has the nice side effect of allowing us to refer to the location of a cell using two bytes: the number of the page (using 0 for the first page and 255 for the last) and the position (or offset) within that page (using 0 for the first position and 255 for the last). This pair of bytes is called the address of a cell and is written as two numbers separated by a colon. For example, the first cell is at address 0:0, the last one is at address 255:255 and the 260th one is at address 1:3 (i.e. the 4th cell of the 2nd page). We will often just write the cell Page:Offset to refer to the cell at address Page:Offset.

The MEMory view is a view on this virtual world. To make the most of the small display of a typical mobile phone, cells are shown on a matrix, but you should always keep in mind that they are in fact disposed in a single row.

The current cell (i.e. the cell we are currently inspecting) is marked by the cursor. The cursor can be moved by using 4 (or left) to go to the previous cell and 6 (or right) to go to the next one. You can also use 2 (or up) and 8 (or down).

Reach the cell 3:25 and then select Edit.

Edit

This screen allows you to change the content of a cell.

Input 56 into the Value field and then select OK.

Memory (56)

The content of the cell has changed. Move the cursor further down until the cell 3:25 is not visible anymore (e.g. until you reach 3:50) and then back until the cell 3:25 is visible again. You can see that the cell has kept the value you have put into it. The cell 3:25, like most of the cells, is a memory cell, that is, a cell that just keeps the value that is put into it.

Select GoTo.

GoTo

This screen allows you to quickly move the cursor to a specific cell.

Input 2 into the Page field, input 40 into the Offset field and then select OK.

Memory (IO)

Cells on page 2 are not memory cells. Instead, they are links to a component called the IO chip. In particular, cells from 2:40 to 2:79 are connected to the display, in this way:

     0  1  2  3  4  5  6  7  8  9
 40  .  .  .  .  .  .  .  .  .  .
 50  .  .  .  .  .  .  .  .  .  .
 60  .  .  .  .  .  .  .  .  .  .
 70  .  .  .  .  .  .  .  .  .  .

Putting a byte from 32 to 126 into one of the cells above will cause a character to appear on the display, according to this table:

     0  1  2  3  4  5  6  7  8  9
 30           !  "  #  $  %  &  '
 40  (  )  *  +  ,  -  .  /  0  1
 50  2  3  4  5  6  7  8  9  :  ;
 60  <  =  >  ?  @  A  B  C  D  E
 70  F  G  H  I  J  K  L  M  N  O
 80  P  Q  R  S  T  U  V  W  X  Y
 90  Z  [  \  ]  ^  _  `  a  b  c
100  d  e  f  g  h  i  j  k  l  m
110  n  o  p  q  r  s  t  u  v  w
120  x  y  z  {  |  }  ~

Memory (88)

Select Edit, input 88 into the Value field and then select OK.

Display (X)

Press # to switch to the outside view.

You can see that the character X is visible on the top left corner of the display. On phones with large displays, the character will likely be more toward the center, since the display of the VM is far smaller than the display of the phone.

Now that we have a better understanding of what "acting on a virtual world" means, we are ready to take another look at the CPU.

Press # again to switch back to the inside view and then 0 to switch to the CPU view.

CPU (BRK)

The CPU can perform a limited number of elementary actions, called instructions or operations. For example: reading a byte from a cell, writing a byte into a cell, adding two bytes together, etc...

A program is essentially a script detailing the sequence of operations that the CPU must perform to produce the desired behaviour.

The CPU retrieves the operations to perform from the cells, starting from the cell 3:0. What for us is just a number like any other, for the CPU has a specific meaning. For example, 0 means BRK (abbreviation of BReaK) for the CPU. The name BRK comes from the fact that, on a real 6502, BRK could be used to suspend the program. This is one of the few differences between a real 6502 and the CPU of the VM. On the CPU of the VM, BRK causes the program to terminate.

Places in the CPU are called registers instead of cells. We will examine only a few of them here. First, there is the PC (or Program Counter). It has room for two bytes and contains the address of the cell containing the next operation to perform. Then there are three registers that have room for a single byte each. They are: the Accumulator (abbreviated by A), X and Y.

Memory (232)

Press 0 to switch to the MEMory view.

Write 232 into the cell 3:0. That is: select GoTo, input 3 into the Page field, input 0 into the Offset field, select OK, select Edit, input 232 into the Value field and then select OK.

Press 0 to switch back to the CPU view.

CPU (INX)

232 means INX (abbreviation of INcrement X) for the CPU. INX causes the X register to be incremented by 1. If the X register already contains 255, it is reset to 0.

Press 1 to advance one step (i.e., let the CPU perform one operation).

CPU (BRK - 2)

You can see that the operation has been carried out: the register X now contains 1 and the PC now points to the next cell (i.e., the cell 3:1, containing the next operation to perform).

We could now change the content of the cell 3:1 and repeat the process again, but this is not the best way to do it. Writing sequences of instructions is better done using the Editor.

Press 1 to advance one step and terminate the program.

Halted

You are given the last chance to have a look at the display, before powering off the VM and returning to the Editor.

Select End.

Empty Prog.

The Editor is a modal editor, i.e. the effects of pressing a key depend on the current mode of operation. There are two major modes of operation (NAVigation and EDiTing) combined with two minor modes of operation (MEMory and ASseMbly).

The NAV MEM mode behaves like the MEMory view described before. Just like before, you can move the cursor using 4 (or left), 6 (or right), 2 (or up) and 8 (or down). The main difference is that you cannot reach the cells beyond the limit of the program. This is because, when writing a program, you are not editing the memory of the VM, but merely writing a template of bytes that will be later used to initialize it.

Prog. (2 232 2 EDT)

Make sure that the cursor is at 3:0 and press 5 (or fire, on some phones).

The EDT MEM mode allows typing the bytes of the program "in place" (i.e. without opening a new screen).

Press 2, *, 2, 3, 2 and 2.

Note the following:

  1. You are not limited to typing one byte, but you can type a whole sequence of bytes.
  2. For bytes with fewer than three digits, you can move to the next byte by pressing *.
  3. For bytes with exactly three digits, moving to the next byte is automatic.

If you make a mistake typing the sequence, you can select Cancel to clear the current byte. If the current byte is already cleared, selecting Cancel will move to the previous byte. Finally, if you reach the first byte and select Cancel, you will return to the NAV MEM mode without making any changes to the program.

Press OK to confirm the changes and return to the NAV MEM mode.

Prog. (2 232 2 NAV)

The first three bytes of the program should now be: 2, 232 and 2. You might recognize 232 as meaning INX, but what about 2?

Press #.

Prog. (ASM - Invalid)

The NAV ASM mode allows you see the program as it would be interpreted by the CPU. You can move the cursor using 2 (or up) and 8 (or down) to scroll the listing, but you cannot press 5 to edit it in place.

So, 2 means ??? for the CPU. Well, that was not very helpful. Let us start the VM (properly this time) to see what happens.

Press *.

INV.OP.

Here is what happened:

  1. The VM was powered on.
  2. The program was copied to the memory cells of the VM.
  3. The CPU read the cell 3:0 to get an operation to perform.
  4. Since 2 has no meaning for the CPU, the VM was halted with an error.

INV.OP. is the abbreviation of "invalid opcode". Opcode is the contraction of "operation code" and is a byte identifying what kind of operation to perform. Some opcodes (e.g. 232) make sense for the CPU, while some others (e.g. 2) do not.

Select End to return to the Editor and then press # to switch to the NAV MEM mode.

Prog. (2 232 2 NAV)

Place the cursor at 3:0, press 5 to switch to the EDT MEM mode and then press # to switch to the EDT ASM mode.

Prog. (EDT)

The EDT ASM mode allows you to lookup a valid opcode.

Press 5 (jkL), 3 (Def) and 2 (Abc).

Prog. (LDA #n)

Before going any further, let me rephrase the sentence:

232 means INX for the CPU.
Without using the misleading word "means" ("means" might suggest that the CPU understands something):
When the CPU reads the opcode 232 from a cell, it increments the X register.

The three-letter label INX is called mnemonic and its only purpose is to remind the programmer of what the CPU does.

Let us have a look at two new opcodes: 169 and 173.

169. When the CPU reads the opcode 169 from a cell, it replaces the content of the Accumulator with the content of the cell after the cell containing the opcode.
173. When the CPU reads the opcode 173 from a cell, it replaces the content of the Accumulator with the content of a specific cell. The address of that cell is computed using, as the offset, the content of the cell after the cell containing the opcode, and, as the page, the content of the cell after the cell providing the offset

The wording is a bit convoluted, but I hope it will become clear with the examples below. What is important right now is to see a similarity between the two opcodes: in both cases, the content of the Accumulator is replaced. The mnemonic for that is LDA (abbreviation of LoaD Accumulator).

Here is an example for the opcode 169:

169 65

Note that, unlike BRK and INX, one byte is not enough to specify the behaviour of the CPU. If we want the CPU to replace the content of the Accumulator, we must specify the new content (65 in this case). In other words: the operation beginning with the opcode 169 is two-bytes long and the second byte (a.k.a. the operand) specifies the new content of the Accumulator.

And here is an example for the opcode 173:

173 40 2

In this case the operation is three-bytes long, and the last two bytes (i.e. the operand) specify the address of a cell where the new value of the Accumulator can be found. In this case, that would be cell 2:40. Yes, the order is inverted and this might be confusing at first, but you will quickly get used to it.

If you look at the two sequences of bytes above, you see that it is not clear that they are doing something similar (i.e. replacing the content of the Accumulator). Furthermore, on a long program composed of several operations, it is difficult to spot where the operations begin and to check if you provided the right number of bytes as operands.

Consider this sequence:

169 40 2

Maybe you meant to load the Accumulator from the cell 2:40. But, of course, the CPU cannot guess what you meant; it would load the Accumulator with 40 and then halt the VM because 2 is not a valid opcode.

Assembly language is an effective method to present the bytes of a program. Every operation is clearly shown in its own line, no matter if it is 1, 2 or 3 bytes long. Its behaviour is unambiguously specified using its mnemonic and, if needed, its operand. The operand itself is formatted using a rigorous pattern. This pattern is a compact way to express the second part of the two sentences above, or, in 6502 jargon, the addressing mode.

Memory view Assembly view Explanation
169 65 LDA #65 LoaD the Accumulator [LDA] with the constant [#n] 65
173 40 2 LDA 2:40 LoaD the Accumulator [LDA] with the content of the cell [n:n] 2:40

The addressing modes here are #n and n:n, where n is the abbreviation of number.

The JBit-QS.pdf sheet contains the full list of the addressing modes of the CPU. You are not expected to understand all of them by the brief descriptions contained in that sheet, but, even if you do not understand them, you can still get some information from the way they look: the number of lower-case characters of an addressing mode is the number of extra bytes an operation needs. For example, the addressing mode n:n,X requires two extra bytes beside the opcode.

Assembly, like every programming language and unlike natural languages, is a formal language and every small detail matters. For example, in English, you might make a punctuation mistake, but you can still be understood. In Assembly, even punctuation is critical. The operations LDA #65 (i.e. 169 65) and LDA 65 (i.e. 165 65) are completely different (the second one, if we ignore the fact that is shorter and faster, is equivalent to LDA 0:65).

Select LDA #n from the list. 169 is inserted, the cursor is moved to the next cell and the mode is switched back to EDT MEM.

Prog. (169 65 EDT)

Press 6, 5 and * to insert the operand.

Press # to switch to the EDT ASM mode.

Press 7 (pqrS), 8 (Tuv) and 2 (Abc).

Prog. (STA n:n)

STA is the mnemonic of STore Accumulator and is in a way the opposite of LDA: it replaces the content of a cell with the content of the Accumulator.

The JBit-QS.pdf sheet contains the full list of the mnemonics of the CPU. Again, you are not expected to understand all of them by the brief descriptions contained in that sheet, but, even without looking at the list, you should already be able to guess what a few other valid mnemonics (LDX, STX, LDY, STY, INY, DEX and DEY) stand for.

Select STA n:n from the list. 141 is inserted, the cursor is moved to the next cell and the mode is switched back to EDT MEM.

Prog. (169 65 141 40 2)

Press 4, 0 and * to insert the first byte of the operand (i.e. the offset).

Press 2 and select OK to insert the second byte of the operand (i.e. the page).

Press # to switch to the NAV ASM mode.

Prog. (ASM - LDA/STA)

Here is the Assembly view of the complete program:

LDA #65
STA 2:40
BRK

To sum up:

  1. The first operation loads the Accumulator with the value 65.
  2. The second operation stores the Accumulator (i.e. 65) to the cell 2:40, causing the character A (see the table above) to appear at the top left corner of the display.
  3. The third operation halts the VM.

Select Debug to test the program.

Debug

Press 1 to advance one step. The Accumulator now contains 65 and the Program Counter 3:2.

Press 1 to advance another step. The Program Counter now contains 3:5. You can check that the character A is visible on the display of the VM by pressing # and then go back to the CPU view by pressing # again.

Select Abort to terminate the program and go back to the Editor.

Save

At the beginning, you are likely to write very short programs and saving your work should not be a priority. In fact, starting from scratch each time might help you to consolidate what you have learned. If in the future you begin writing larger programs, here is how you can save them:

Select Save, type Tutorial and select OK.

Main Menu (Store)

Select Back followed by Exit to terminate JBit, and then start JBit again.

JBit is a modular system and can be configured to include more or fewer tools, depending on the limitations of your phone. The main menu lists which tools have been included.

In this tutorial I am using JBit1M, a version of JBit targeting old phones that comes with only two tools: the Store and the Editor. We have already used the Editor and below we are going to use the Store, but before that, let me introduce a couple of tools that you probably have in your version of JBit:

Demos
This tool provides a few ready-made programs that you can study. In particular, the 6502 demos are especially designed for the beginning programmer. If you have problem installing a version of JBit including the Demos tool, you can type the 6502 demos manually using the JBit-E0.pdf sheet as a reference.
Paint
This tool allows you to create and edit simple images with JBit. It was designed to show how to write tools and it is not very advanced, but it might be useful nevertheless.

An important thing to understand is that the tools operate on the current program. Unlike most computer applications, JBit can only work on one "document" at a time; if you run a demo or load a program, the program you are working on will be silently replaced, even if it has not been saved.

You might wonder why there is a Paint tool, if the display of the VM cannot show images. The reason is that you can equip the VM with different versions of the IO chip:

MicroIO
The MicroIO version provides only the most basic facilities to write interactive programs.
MIDP1
The MIDP1 version adds various functionalities, but, most importantly, adds the ability to display images.
MIDP2
The MIDP2 version adds the ability to write 2D action games.

The MicroIO version should not be dismissed. You can learn it quickly and it so simple that you do not need to consult a reference to use it. Even if you have a better version available, targeting the MicroIO version when you write your programs is a very good idea.

Select Store.

Store

The Store tool gives you a container where you can keep your programs for future editing. You have already used the Store tool without realizing it, when you saved the program from within the Editor, but accessing it manually gives you more options. The list of the saved programs is presented to you and you can use a few commands to manage them.

Load&Edit (the default command) loads a program and starts the Editor. Load&Run loads a program and starts the VM. Load just loads a program.

Save asks for the name of the current program and saves it. You cannot overwrite an existing program using Save; for that you have to use Overwrite.

Info, Copy, Rename and Delete do pretty much what you expect them to do. They do not change the current program.

When you upgrade JBit to a new version, your phone should ask you if you want to keep its data and it is important to reply YES to keep your saved programs. The exact wording of the question varies from phone to phone.

Select Tutorial.

Prog. (162 88 142 40 2)

Here is a quick editing session, as a review:

  1. Press 5 to start editing.
  2. Press # to lookup the first opcode.
  3. Type 539 (for LDX).
  4. Select LDX #n from the list.
  5. Type 88* to enter the operator.
  6. Press # to lookup the second opcode.
  7. Type 789 (for STX).
  8. Select STX n:n from the list.
  9. Select OK to confirm the changes (the result should be: 162 88 142 40 2).
  10. Press * to run the program.
  11. Select Video to have a look at the display.
  12. Press any key (e.g. the soft key you have used for selecting Video, even if it does not have a label attached to it anymore) to go back to the HALTED screen.
  13. Select End to go back to the Editor.
  14. Select Save to save the new version of the program to the Store.

Select Back and then Exit to terminate JBit.