RSASM (Really Specific Assembly)

An open-source assembler for a really specific custom 8-bit cpu.

Installation

• Build the project using premake
• Install bin/release/assembler(.exe) in your path or wherever you want
• Happy coding !

CLI

The CLI can be used with one of this two ways : - assembler (informational_flags) - assembler source_file_path (flags)

For the second variant, you must place the source_file_path first.

CPU architecture

The CPU is composed of 6 8-bits general purpose registers (a - f), an hl 16-bits register (h is the high part, l the low part) that is used to point to the 64kib RAM.

The stack proposed by RSASM is purely virtual. Thus, a choice had to be made concerning the stack pointer. The register f is choosen to be the stack pointer. Any stack-related operation will modify e. CALL will modify a too and RET d.

Function's paramaters are passed, in order, to b, c and d. If more paramaters are necessary, they are passed by the stack with the fourth paramater first pushed and the last paramater lastly pushed (on top of the stack). The return value of functions (if any) is passed through a.

Functions can and will modify registers. It is the responsability of the caller to save register's state to the RAM or stack (if needed) before calling a function.

Assembly language

Here lay the documentation of RSASM instructions :

opcode	src argument	dst argument	effect
NOP			set the current instruction to 0
ADD	lreg, rreg		lreg = lreg + rreg
SUB	lreg, rreg		lreg = lreg - rreg
LTEQ	lreg, rreg		lreg = lreg <= rreg
GTEQ	lreg, rreg		lreg = lreg >= rreg
LT	lreg, rreg		lreg = lreg < rreg
GT	lreg, rreg		lreg = lreg > rreg
EQ	lreg, rreg		lreg = lreg == rreg
AND	lreg, rreg		lreg = lreg & rreg
OR	lreg, rreg		lreg = lreg | rreg
XOR	lreg, rreg		lreg = lreg ^ rreg
NOT	reg		reg = ~reg
ADDS	lreg, rreg	dreg	dreg = lreg + rreg
SUBS	lreg, rreg	dreg	dreg = lreg - rreg
LTEQS	lreg, rreg	dreg	dreg = lreg <= rreg
GTEQS	lreg, rreg	dreg	dreg = lreg >= rreg
LTS	lreg, rreg	dreg	dreg = lreg < rreg
GTS	lreg, rreg	dreg	dreg = lreg > rreg
EQS	lreg, rreg	dreg	dreg = lreg == rreg
ANDS	lreg, rreg	dreg	dreg = lreg & rreg
ORS	lreg, rreg	dreg	dreg = lreg | rreg
XORS	lreg, rreg	dreg	dreg = lreg ^ rreg
NOTS	reg	dreg	dreg = ~reg
LDX	reg		reg = value of next byte
RI	sreg	dreg	dreg = IO[sreg] DOES NOTHING RIGHT NOW
CARRY		dreg	dreg = carry bit
WI	lreg, rreg		IO[lreg] = rreg DOES NOTHING RIGHT NOW
COPY	sreg	dreg	dreg = sreg
JHL			pc = hl
JIHL	sreg		if sreg == 0xff then pc = hl
JZHL	sreg		if sreg == 0 then pc = hl
JCHL			if carry bit == 1 then pc = hl
MWRITE	sreg		RAM[hl] = sreg
MREAD		dreg	dreg = RAM[hl]
BYTE	number		set the current instruction to number
MOV	number	dreg	dreg = number
PUSH	sreg		stack.push(sreg)
POP		dreg	dreg = stack.pop()
CALL	tag		push pc; jmp tag
RET			pop pc
JMP	tag		pc = addr(tag)
JI	tag, sreg		if sreg == 0xff then pc = addr(tag)
JZ	tag, sreg		if sreg == 0 then pc = addr(tag)
JC	tag		if carry bit == 1 then pc = addr(tag)