MEGA-CHIP8 v1.0b, by: Martijn Wenting / Revival Studios #

A modified version of the CHIP-8 game interpreter originally made by Andreas Gustafsson / Erik Bryntse (schip).

MEGA-CHIP offers:

• 256x192 resolution
• Indexed coloring (255 colors max + transparency)
• Fixed high-speed speed in megachip mode
• Custom sprite sizes
• Update timing at ClearScreen
• Extended I-register range (24 bit addessing, 32MB max)
• Digitised sound (mono 8bit)
• Downward compability (you can run your old CHIP/S-CHIP games)
• Spritecolor 0 = transparent.
• Spritecollision will occur if (backgroundcolor>0) when plotting spritepixel.

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 pro- cessor 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 corre- sponds to the leftmost pixel. All drawing is done in XOR mode. If this causes one or more pixels to be erased, VF is <> 00, other- wise 00.

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

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.

MCHIP8 Technical specs (for implementation): #

• CLS will be used for timing/flipping/noflickering purpose
• Emulationspeed/Num.ticks per frame will be fixed (1000 ticks????) OR timed to CLS
• LDHI I,nnnnnnn instruction allows 24-bit adressing, always follow LDHI by a NOP instruction.
• LDPAL Will load 32-bit colors to an internal mchip8 pallete at position 1. Color 0 is always black/transparent.
• SPRW/SPRH can be 0..255 , use 0 for a width/height of 256
• SPRW/SPRH will overwrite n-setting for non-character sprites.
• ALPHA/FADE will effect the fade-factor of the screenbuffer (0..255)

Instructions marked with (+) are new in MEGA-CHIP.

0010+    Disable Megachip mode       (MEGAoFF)
0011+    Enable Megachip mode        (MEGAON)
01nn+    I=(nn<<16)+nnnn , PC+=2;    (LDHI  I,nnnnnn , always follow LDHI with a NOP)
02nn+    Load nn-colors palette at I (LDPAL nn)
03nn+    Set Sprite-width to nn      (SPRW  nn)
04nn+    Set Sprite-height to nn     (SPRH  nn)
05nn+    Set Screenalpha to nn       (ALPHA nn, will become FADE nn)
060n+    Play digitised sound at I   (DIGISND), will add n for loop/noloop
0700+    Stop digitised sound        (STOPSND)
080n+    Set sprite blendmode        (BMODE n) (0=normal,1=25%,2=50%,
                                                3=75%,4=addative,5=multiply)
00BN+    Scroll display N lines up   (SCRU n)

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

F0nn     I:= 28bit addres
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
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)
FX30*    Point I to 10-byte font sprite for digit VX (0..9)
FX75*    Store V0..VX in RPL user flags (X <= 7)
FX85*    Read V0..VX from RPL user flags (X <= 7)