CHIP-8 is an easy-to-learn programming language that lets you write your own programs. To use the CHIP-8 language, you must first store the 512-byte CHIP-8 language program at memory locations 0000 to 01FF. The CHIP-8 language program is shown in Appendix C in hex form so you can enter it directly in memory using the hex keyboard. You can then record it on a memory cassette for future use. Each CHIP-8 instruction is a two-byte (4-hex-digit) code. There are 31, easy-to-use CHIP-8 instructions as shown in Table L
When using CHIP-8 instructions your program must always begin at location 0200. There are 16 one-
byte variables labeled 0-F. VX or VY refers to the value of one of these variables. A 63FF instruction sets variable 3 to the value FF (V3=FF). I is a memory pointer that can be used to specify any location in RAM. An A232 instruction would set I= 0232. 1 would then address memory location 0232.
Branch Instructions
There are several types of jump or branch instructions in the CHIP-8 language. Instruction 1242 would cause an unconditional branch to the instruction at memory location 0242. Instruction BMMM lets you index the branch address by adding the value of variable 0 to it before branching. Eight conditional skip instructions let you test the values of the 16 one-byte variables or determine if a specific hex key is being pressed. This latter capability is useful in video game programs. (Only the least significant hex digit of VX is used to specify the key.)
A 2570 instruction would branch to a subroutine starting at location 0570. 00EE at the end of this subroutine will return program execution to the
instruction following the 2570. The subroutine itself could use another 2MMM instruction to branch to (or call) another subroutine. This technique is known as subroutine nesting. Note that all subroutines called (or branched to) by 2MMM instructions must end with 00EE. Ignoring this rule will cause hard-to-find program bugs.
How to Change and
Use the Variables
The CXKK instruction sets a random byte value into VX. This random byte would have any bits matching 0 bit positions in KK set to 0. For example, a C407 instruction would set V4 equal to a random byte value between 00 and 07.
A timer (or real-time clock) can be set to any value between 00 and FF by a FX15 instruction. This timer is automatically decremented by one, 60 times per second until it reaches 00. Setting it to FF would require about 4 seconds for it to reach 00. This timer can be examined with a FX07 instruction. A FX18 instruction causes a tone to be sounded for the time specified by the value of VX. A value of FF would result in a 4-second tone. The minimum time that the speaker will respond to is that corresponding to the variable value 02.
A FX33 instruction converts the value of VX to decimal form. Suppose 1=0422 and V9=A7. A F933 instruction would cause the following bytes to be stored in memory:
0422 01
0423 06
0424 07
Since A7 in hex equals 167 in decimal, we see that the
