SUPER-CHIP v1.1 #

… a modified version of the CHIP-8 game interpreter originally made by Andreas Gustafsson.

S-CHIP offers:

• full screen resolution in new extended screen mode
• downward compability (you can run your old CHIP games)
• higher speed in extended mode
• a larger 16x16 sprite available
• new, larger fonts for scores
• you can pass information to and from a S-CHIP program
• programmable exit from the S-CHIP interpreter possible
• no need to turn off the clock
• it will always start
• instructions to scroll the screen down, left, and right

Introduction to CHIP #

For those who don’t remember, the CHIP-8 programming language was used in a number of home computers based on RCA’s CDP1802 processor in the late 1970’s. It’s a small, interpreted language designed specifically for writing simple video games. It has less than 40 instructions, including arithmetic, control flow, graphics, and sound. The instructions are all 16 bits long and are executed by a very compact virtual machine interpreter (the 1802 implementation was 512 bytes long).

Technical specification #

The CHIP-8 virtual machine is byte-addressable and has an address space of 4 kB at addresses 000-FFF hex. However, addresses 000-1FF are reserved (this is where the CHIP-8 interpreter used to reside). Therefore, the CHIP-8 program itself begins at address 200. All instructions are 16 bits long and by convention instruc- tions always start at an even address.

The machine has 16 8-bit general-purpose registers called V0..VF. The VF register is modified by certain instructions and works as a carry flag and sprite collision indicator. There is also a 16-bit pointer register I (though only the low 12 bits are generally used).

A CHIP-8 program displays graphics by drawing sprites that are 8 pixels wide and 1..15 pixels high. The screen resolution is 32 by 64 pixels in standard mode. The origin is the upper left corner of the screen, and all coordinates are positive. The sprites are stored in bigendian format, i.e., the most significant bit corresponds to the leftmost pixel. All drawing is done in XOR mode. If this causes one or more pixels to be erased, VF is <> 00, otherwise 00.

In extended screen mode the resolution is 64 by 128 pixels. A sprite of 16x16 size is available.

NOTE: This is written strangely but it means 128 columns and 64 rows.

There are two timers: the delay timer and the sound timer. Both timers count down about 60 times per second when nonzero; the speaker will beep whenever the sound timer is nonzero.

In the instruction table below, NNN is a 12-bit address, KK is an 8-bit constant, and X and Y are 4-bit register numbers. Hex characters represent themselves. The two first characters of the instruction code form the lower-address byte of the instruction, the first character being the more significant nibble.

Instructions marked with (*) are new in SUPER-CHIP.

00CN* Scroll display N lines down
00E0  Clear display
00EE  Return from subroutine
00FB* Scroll display 4 pixels right
00FC* Scroll display 4 pixels left
00FD* Exit CHIP interpreter
00FE* Disable extended screen mode
00FF* Enable extended screen mode for full-screen graphics
1NNN  Jump to NNN
2NNN  Call subroutine at NNN
3XKK  Skip next instruction if VX == KK
4XKK  Skip next instruction if VX <> KK
5XY0  Skip next instruction if VX == VY
6XKK  VX := KK
7XKK  VX := VX + KK
8XY0  VX := VY, VF may change
8XY1  VX := VX or VY, VF may change
8XY2  VX := VX and VY, VF may change
8XY3  VX := VX xor VY, VF may change
8XY4  VX := VX + VY, VF := carry
8XY5  VX := VX - VY, VF := not borrow
8XY6  VX := VX shr 1, VF := carry
8XY7  VX := VY - VX, VF := not borrow
8XYE  VX := VX shl 1, VF := carry
9XY0  Skip next instruction if VX <> VY
ANNN  I := NNN
BNNN  Jump to NNN+V0
CXKK  VX := pseudorandom_number and KK
DXYN* Show N-byte sprite from M(I) at coords (VX,VY), VF := collision. If N=0 and extended mode, show 16x16 sprite.
EX9E  Skip next instruction if key VX pressed
EXA1  Skip next instruction if key VX not pressed
FX07  VX := delay_timer
FX0A  wait for keypress, store hex value of key in VX
FX15  delay_timer := VX
FX18  sound_timer := VX
FX1E  I := I + VX
FX29  Point I to 5-byte font sprite for hex character VX
FX30* Point I to 10-byte font sprite for digit VX (0..9)
FX33  Store BCD representation of VX in M(I)..M(I+2)
FX55  Store V0..VX in memory starting at M(I)
FX65  Read V0..VX from memory starting at M(I)
FX75* Store V0..VX in RPL user flags (X <= 7)
FX85* Read V0..VX from RPL user flags (X <= 7)

Notes on the HP48SX implementation #

CHIP-8 programs are stored in the HP48SX as string objects containing the bytes of the program in increasing address order, beginning with the byte at 0200. The interpreter itself is a machine code object that should be run with the CHIP-8 program string on level 1. 4 kB of free memory is needed. If an error is detected during execution, the address of the current CHIP-8 instruction is returned as a binary integer on level 1.

To quit, press the backspace key. Pressing ENTER restarts the CHIP-8 program, and the +/- key turns the sound off or on.

Subroutine nesting is limited to 16 levels.

Most chip-8 programs are written for a 16-key hex keyboard with following layout:

1 2 3 C                                           7 8 9 /
4 5 6 D  This keyboard is emulated on the HP48SX  4 5 6 *
7 8 9 E  using the following keys:                1 2 3 -
A 0 B F                                           0 . _ +

This may cause some confusion with programs requiring numerical input.

Copyrights, etc #

Parts of this document is copied from the original CHIP-48 documentation written by Andreas Gustafsson. Below is the original copyright message for CHIP-48 v2.25

(c) Copyright 1990 Andreas Gustafsson

Noncommercial distribution allowed, provided that this copyright message is preserved, and any modified versions are clearly marked as such.

The program makes use of undocumented low-level features of the HP48SX calculator, and may or may not cause loss of data, excessive battery drainage, and/or damage to the calculator hardware. The author takes no responsibility whatsoever for any damage caused by the use of this program.

THIS SOFTWARE IS PROVIDED “AS IS” AND WITHOUT ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

The modifications from CHIP v2.25 to S-CHIP v1.1 is made by

Erik Bryntse