cstrotm / ch8asm

Chip-8 Assembler in Forth

CHIP-8 Assembler

Intro

This is an very early version of a CHIP-8 Assembler based on VolksForth. The current version is being created for the Atari 8bit, but it should be easy be ported to other VolksForth or Forth implementations.

Usage

The CHIP-8 Assembler is started by executing the file ch8asm.com. It used the commandline interface of VolksForth.

CHIP-8 Assembler code can be entered directly from the keyboard or loaded from a file.

To load from a file, enter

ASM <filespec>

The assembled binary code can be saved to file with

SAVEBIN <filespec>

To leave the Assembler to DOS enter the command

DOS

By default, the Assembler starts in HEX mode, all numbers are interpreted as hexadecimal numbers. The mode can be switch with the commands decimal (switched to decimal mode) and hex (switched to hexadecimal mode)

All numbers can be prefixed to denote the number base:

&100 - a decimal 100 $100 - a hexadecimal 100 %100 - a binary 100

Memnonics

This assembler is using a postfix notation where the parameters of a CHIP-8 command are written first, followed by the command itself.

Take a look at the example source code files distributed with this software to get a feel for the syntax.

• NNN ORG - Set the assemble address to NNN
• CLS - CLS memnonic - clear screen
• RET - RET memnonic - return from subroutine
• NNN JMP - JMP memnonic - jump to address
• NNN JSR - JSR Memnonic aka CALL - jump to subroutine at address
• VX NN SEQ - Skip next command if VX is equal to NN
• VX NN SNE - Skip next command if VX is not equal to NN
• VY VX SRE - Skip next command if VY is equal to VX
• VX NN LDV - Load immediate value NN into register VX
• VX NN ADV - Add immediate value NN to register VX
• VY VX CPR - Copy register vy into vx
• XY VX ADD - Set VX equal to VX plus VY. In the case of an overflow, VF is set to 1. Otherwise 0.
• VY VX SUB - Set VX equal to VX minus VY. In the case of an underflow, VF is set 0. Otherwise 1. VF = VX > VY
• VY VX ORR - Set VX equal to the bitwise or of the values in VX and VY
• VY VX AND - Set VX equal to the bitwise and of the values in VX and VY
• VY VX XOR - Set VX equal to the bitwise xor of the values in VX and VY
• VY VX SHRV
• VX SHR - Set VX equal to VX bitshifted right 1. VF is set to the least significant bit of VX prior to the shift. Originally this opcode meant set VX equal to VY bitshifted right 1 but emulators and software seem to ignore VY now.
• VY VX SBR - Set VX equal to VY minus VX. VF is set to 1 if VY > VX. Otherwise 0
• VY VX SHLV VX SHL - Set VX equal to VX bitshifted left 1. VF is set to the most significant bit of VX prior to the shift. Originally this opcode meant set VX equal to VY bitshifted left 1 but emulators and software seem to ignore VY now
• VY VX SNEV - Skip the next instruction if VX does not equal VY
• NNN LDI - Set I equal to NNN
• NNN JPV - Set the PC to NNN plus the value in V0
• NN VX RND - Set VX equal to a random number ranging from 0 to 255 which is logically anded with NN
• NN VX VY DRW - Display N-byte sprite starting at memory location I at VX, VY. Each set bit of xored with what’s already drawn. VF is set to 1 if a collision occurs. 0 otherwise.
• VX SKP - Skip the following instruction if the key represented by the value in VX is pressed.
• VX LDT - Set VX equal to the delay timer.
• VX LKY - Wait for a key press and store the value of the key into VX.
• VX SDT - Set the delay timer DT to VX.
• VX SST - Set the sound timer ST to VX.
• VX AVI - Add VX to I. VF is set to 1 if I > 0x0FFF. Otherwise set to 0.
• VX LDF - Set I to the address of the CHIP-8 8x5 font sprite representing the value in VX.
• VX BCD - Convert that word to BCD and store the 3 digits at memory location I through I+2. I does not change.
• VX STR - Store registers V0 through VX in memory starting at location I. I does not change.
• VX LDR - Copy values from memory location I through I + X into registers V0 through VX. I does not change.
• NN DB - store a singe byte in memory
• LABEL: <label> - create label ( for jumps and sprites )

Limits

• Not fully tested, there are bugs
• No forward references - labels must be declared before use
• Only CHIP-8, no extended CHIP-8 machine support
• Assembler must be restarted after an assembly run

Future development

• Forward reference label
• SCHIP-8 and CHIP-8 extensions support
• Debugger/Disassembler