Chip-8 on the COSMAC VIP: Initialisation

This post is part of a series on the Chip-8 interpreter. In this post I will look at how the Chip-8 interpreter is launched and initialised on the COSMAC VIP. If you are unfamiliar with the RCA CDP1802 processor, you might want to read this earlier post first. In a previous post I talked about how the VIP’s ROM is executed. The ROM basically provides four functions:

1. The user can enter bytes to be written to memory starting at a selected address
2. The user can inspect the memory at a selected address to see what is stored there
3. The user can save a block of memory to tape starting from a selected address
4. The user can restore a block of memory from tape to start at a selected address

That’s it! To access these functions, the user must hold the C key down as they flip the RUN switch on the VIP. If they flip RUN without holding the C key the ROM will do just three things. First it will enable the RAM, then it will find the top of available RAM to determine how much memory the system has, then it will execute the programme stored in RAM at 0x0000.

So it is expected that the user has entered and verified the Chip-8 interpreter programme using functions 1 and 2, saved it for further use with function 3 and then used function 4 each time they want to run the interpreter.

Whenever any programme is first run on the VIP, the registers are configured as follows:

• X is 0, so R0 will be used for indirect register addressing operations.
• P is also 0, so R0 is also the program counter.
• R0 points to the start of RAM (0x0000).
• R1 points to the last byte of on-card RAM (note that this might not be the last byte of all the RAM, if an external RAM module has been installed).
• The remaining registers will be in an undefined state.

Another important thing to note is that the VIP’s display will be off at this point, so no screen refresh interrupts will be generated.

Chip-8 uses the general purpose 16-bit registers in different roles as follows:

I’ll explain these in more detail as they become relevant. One initial thing to note though, is that the roles chosen for each register are not arbitrary. Whenever DMA is used, as it is by the display controller in the COSMAC VIP, R0 is always used as the pointer to the memory to be accessed. Also whenever interrupts are enabled, as they will be when the display is switched on, R1 should always be set as the program counter for the interrupt routine and should initially be pointing to the first byte of that routine. Here’s a flowchart showing how the interpreter sets these up.

Here’s the code for this section of the interpreter.

This sequence is run just once, when the interpreter first starts. The whole thing takes 40 machine cycles. One machine cycle on the CDP1802 requires 8 clock cycles. So, on the COSMAC VIP, which has a clock speed of 1.7609Mhz, one machine cycle takes 4.54 micro seconds. This equates to an execution time of 181.6 micro seconds for this routine.

For a programmer of a contemporary interpreter, the initialise sequence is unlikely to follow this pattern, although the basic tasks, such as initialising the stack pointer, will still need to be carried out.

In the next post in this series, I”ll look at the Chip-8 call, or fetch and decode, sequence in detail.

For other posts in this series, refer to the post index or instruction index.