VERA Overview

VERA stands for Versatile Embedded Retro Adapter and consists of (copy from official documentation):

  • Video generator featuring:
    • Multiple output formats (VGA, NTSC Composite, NTSC S-Video, RGB video) at a fixed resolution of 640x480@60Hz
    • Support for 2 layers, both supporting either tile or bitmap mode.
    • Support for up to 128 sprites.
    • Embedded video RAM of 128kB.
    • Palette with 256 colors selected from a total range of 4096 colors.
  • 16-channel Programmable Sound Generator with multiple waveforms (Pulse, Sawtooth, Triangle, Noise)
  • High quality PCM audio playback from an 4kB FIFO buffer featuring up to 48kHz 16-bit stereo sound.
  • SPI controller for SecureDigital storage.

In this short introduction post we will discuss access to VERA and write some very simple examples. We will explain the memory organization and test it with Basic and later translate it into Assembly program. At the end is breakdown with links to detailed descriptions of many of VERA features.

When programming in Basic we have few handy commands that make access to graphics simple and seamless. For example using VPOKE and VPEEK we can write and read to and from any address in video memory. With VLOAD we can load chunks of data directly into Video memory. So why would accessing it with machine code be any different?

To explain let’s look at the (simplified) memory map of Commander X16. This illustration actually describes the most important concept in programming VERA, which is the fact that to do anything with it we can only access it through 32 bytes located in memory locations $9F20 - $9F3F:

The CPU is using 16 bit address bus and therefore can directly access 64 Kbytes of memory. We call it CPU memory. Commander X16 actually has more memory which CPU can access through “banking switching” however it can still only directly address 64K at any given time.
On the other hand we have 128K of Video memory (VRAM). That memory is completely separated and isolated from CPU memory therefore CPU has no direct access to it. The only way to manipulate the data in Video memory and therefore the content of the display is through a fairly narrow window in the CPU memory.
Currently 32 bytes are reserved for VERA communication and those 32 bytes contain 36 VERA registers. To be able to access 36 registers in only 32 addresses four of the registers have dual functionality that can be switched by changing the DCSEL bit in register 5.
Let’s look at those 36 bytes or registers that we can use:

Register Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 $9F20 Address Lo VRAM Memory address bits 0-7
1 $9F21 Address Mid VRAM Memory address bits 8 - 15
2 $9F22 Address Hi Address Increment Dec - Bit 16
3 $9F23 Data 0 Data Register 0
4 $9F24 Data 1 Data Register 1
5 $9F25 Control Reset - DC Address Select
7 $9F27 ISR Sprite Collissions AFLOW SPRCOL LINE VSYNC
8 $9F28 IRQ Line IRQ bits 0-7
9-0 $9F29 Video Current Field Sprite Enable Layer 1 Enable Layer 0 Enable - Chroma Disable Output Mode
10-0 $9F2A Horizontal Scale Active Display Horizonatl Scale
11-0 $9F2B Vertical Scale Active Display Vertical Scale
12-0 $9F2C Border Color Border Color
9-1 $9F29 Horizontal Start Active Display Horizontal Start Bits 2 - 9
10-1 $9F2A Horizontal Stop Active Display Horizontal Stop Bits 2 - 9
11-1 $9F2B Vertical Start Active Display Vertical Start Bits 1 - 8
12-1 $9F2C vertical Stop Active Display Vertical Stop Bits 1 - 8
13 $9F2D Layer 0 - Config Map Height Map Width T256C Bitmap Mode Color Depth
14 $9F2E Layer 0 - Mapbase Map Base Address Bits 9 - 16
15 $9F2F Layer 0 - Tilebase Tile Base Address Bits 11 - 16 Height Width
16 $9F30 Layer 0 - H Scroll Horizontal Scroll Bits 0 - 7
17 $9F31 Layer 0 H Scroll - Horizontal Scroll Bits 8 - 11
18 $9F32 Layer 0 - V Scroll Vertical Scroll Bits 0 - 7
19 $9F33 Layer 0 - V Scroll - Vertical Scroll Bits 8 - 11
20 $9F34 Layer 1 - Config Map Height Map Width T256C Bitmap Mode Color Depth
21 $9F35 Layer 1 - Mapbase Map Base Address Bits 9 - 16
22 $9F36 Layer 1 - Tilebase Tile Base Address Bits 11 - 16 Height Width
23 $9F37 Layer 1 - H Scroll Horizontal Scroll Bits 0 - 7
24 $9F38 Layer 1 H Scroll - Horizontal Scroll Bits 8 - 11
25 $9F39 Layer 1 - V Scroll Vertical Scroll Bits 0 - 7
26 $9F3A Layer 1 - V Scroll - Vertical Scroll Bits 8 - 11
27 $9F3B Audio Control FIFO Full/Reset - 16 Bit Stereo PCM Volume
28 $9F3C Audio Rate PCM Sample Rate
29 $9F3D Audio Data Audio FIFO Write Only Data Register
30 $9F3E SPI Data SPI (Serial Peripheral Interface) Data Register
31 $9F3F SPI Control Busy - Slow Clock Select

First three registers are used to set address into VRAM. We have 17 bits at our disposal (0 – 16). That gives us a range from $00000 to $1FFFF or 128 Kbytes. Of course it is no coincidence that VPOKE command has the same range. 
Not all VRAM memory is created equal. As already marked in the picture above there are three sections at the end of the memory space with special role:

Address Range Description
$00000 - $1F9BF Video RAM for Maps and Tiles
$1F9C0 - $1F9FF Programmable Sound Generator Registers (64 bytes)
$1FA00 - $1FBFF Color Palette
$1FC00 - $1FFFF Sprite Attributes (1024 bytes, 128 sprites - 8 bytes each)

We see that video memory is between $00000 and $1FFFF and that we have dedicated space for Palette, sprite registers and Programmable Music Generator from $1F9C0 on. That means that to be able to manipulate VRAM we need 17 bits, which is exactly what we have in registers 0-2. Register 0 contains bits 0-7, Register1 contains bits 8-15 and Register 2 contains bit 16 and some other settings we look at later.

After setting the address we can use Data registers to write or read from Video Memory. We can use Register 3 ($9F23) as Data Register 0 and Register 4 ($9F24) as Data Register 1. We can choose which one we prefer by setting Bit 0 in Register 5 ($9F25) with value 0 selecting Data Register 0 and value 1 selecting Register 1. Note that Bit 7 in this register resets the VERA settings but I recommend not to use it from BASIC.
Next we have to explain probably the most important concept of using VERA and accessing Video Memory. Register 2 ($9F22) contains highest bit of address but also Increment and bit to switch between Increment and Decrement.

Increment is setting that defines automatic increment/decrement of VERA address in registers 0-2 after every read or write. Since we have 4 bits we have 16 different values but what does that actually mean.
The best way to describe how increment works let’s imagine following example and test it. As you remember default video mode of Commander X16 after start (or reset) is 80 column text mode. The displayed text starts in Video memory $00000 which we can verify by VPOKE to that address. The memory is organized in a way that first byte contains a character code. The second character contains a color attribute, the third second character displayed, fourth color attribute for that character and so on. So if we want to write only characters we have to write to every other address starting with $00000, then $00002 followed by $00004 and so on. Obviously the address has to be incremented by two for each write to the screen. And that is exactly what the Increment setting does automatically.
So let’s test this theory and write a simple BASIC program to write to the video memory without using VPOKE but just POKE to the VERA external registers and see if we can recreate above scenario.
First step is to decide which data register we will use. Let’s just use Register 0. We do that with

POKE $9F25,0

Next we have to set screen address to $00000 and set Increment to 2 so we can write to every other address. That means we set Low and Mid byte to 0 and Hi Byte to $20 Hex.

POKE $9F20,0:POKE$9F21:POKE $9F22,$20

If we write some value to Data Register 0 ($9F23) it should display the character in the top left corner and increment the address by two. To make it more interesting let’s put all this into a program:

The Increment values are not simply represented by binary value from 0 – 15 but they have the following values to increase the reach of the Increment:

Increment Setting Actual Increment Amount
0 or $0 0
1 or $1 1
2 or $2 2
3 or $3 4
4 or $4 8
5 or $5 16
6 or $6 32
7 or $7 64
8 or $8 128
9 or $9 256
10 or $A 512
11 or $B 40
12 or $C 80
13 or $D 160
14 or $E 320
15 or $F 640

Knowing this we can modify the above program and try to write vertically. If we recall in the default mode each line takes 256 bytes of memory and only part is visible. 80 out of 128 characters and attributes. To write vertically we therefore have to increment memory location pointer by 256 after every write. Based on a table above we have to use value 9 for increment and now our code looks like this:

No real surprises there. We could use what we learned in Basic and in special cases we might be able to speed up the code a little but not in significant way, especially not in real life scenarios.

Everything we did above we can also do in reverse by using the Decrement bit 3 in Register 2. If it is set the Increment works as Decrement for the same value.


The logical next step is to use the above learned in assembly. Since POKE command is closest we get to hardware from Basic the translation is very simple. Let’s look at below Assembly code and walk through it. Commander X16 comes with Monitor built in which is very convenient. It allows us write simple assembly programs right there on the system without complicated setup and external tools or installing additional tools on a system itself.

First three lines of code simply write 0 into registers at $9F25, $9F20 and $9F21. Mnemonic STZ (STore Zero) was added to 65C02 so if you try to use it on Commodore 64 or any other 6502 computer it will not be recognized.
Lines 4 and 5 first stores value $20 hex into Accumulator, which is the main register in the CPU and store it into $9F22 essentially setting the Memory address to $00000 and setting increment to 2.
We will use Accumulator for two things, to store the value of the next character to be written to screen and as a counter to write exactly 80 characters. Since we will start with A, which has screen code 1 (just like in the Basic program) we initialize it by setting it to 1 in Line 6.
Lines 7-10 are our loop. We write 1 to first location in video memory which in first iteration is $00000. Then in line 8 we increment Accumulator by one. In line 9 we compare it with value $51 hex, which is 81 in decimal. If we would compare with 80 the loop would exit too soon (we could use BPL but let’s keep it simple for now). Then in line we check the result of comparison and if the comparison was not equal we jump back to the beginning of the loop to line 7 (address $4010).
During second iteration therefore we write 2 into address $00002, then 3 into $00004 and so on until we write all 80 characters.
In line 11 we Return from Subroutine and go back to basic or wherever we called this function from.
Like every piece of code this routine could be written in many different ways but I feel this is the clearest way to do it.
Before we wrap up let’s quickly look at how to write it and test it. At the end of this part our screen should look similar to this:

We have to start Monitor by writing command MON
Our cursor will be waiting after the prompt in the form of dot.
Without any additional preparation we can start writing code by telling Monitor we will write assembly and at what address by using command A immediately followed by mnemonic of the first assembly command and parameter (if any). In our case we just type:

A4000 STZ $9F25

After that monitor will expand the line and show the actual bytes stored in memory at locations $4000 - $4002 like we see on above screenshot and wait for next command in next line with new address $4003. So we just keep typing the whole program and when finished on memory location $4019 we simply press enter and we will be at dot prompt again. By typing X we can exit monitor.
All we have to do now is call the program by calling it from Basic by:

SYS $4000

And we should see 80 characters magically appear in the first line of the screen.

Have fun experimenting with and changing your first assembly program.

Download Printable Vera Registers Reference

Further reading related to VERA:


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