BSS

BSS is an assembly-style esoteric programming language created by User:CatLooks. It has a lot of common with 6502 Assembly and requires source code to be compiled for execution.

Limitations

There are some limitations in BSS:

• Compiled File (ROM) must be less/equal than 32Kb.
• RAM is 8192 bytes.
• WRM is 16384 bytes.
• Stack is limited to 128 addresses.
• All registers are 8-bit (with some exceptions).

Execution

Registers

BSS has 7 registers to work with:

• A - Accumulator
• B - Buffer
• X - Index
• I - Instruction Pointer
• P - Status
• S - Stack Pointer
• D - Destination Index

Address Space

BSS has 16-bit address space, which is divided into sections:

• $0000 - $1FFF RAM (where you store all your calculations)
• $4000 - $7FFF WRM (same as RAM, but you can save its state via savefile)
• $2000 - $3FFF Register Space
• $0100 - $01FF Stack (located in RAM)
• $8000 - $FFFF ROM (your code file, filler byte is $00)

Opcodes

BSS has 32 opcodes with values $00 - $1F. Codes used here:

• # - direct byte
• a - address
• a, x - address shifted by X
• [d], x - indirect destination shifted by X

Syntax	Opcode	Size	Description
lda #	$00	2	Load a byte to A
lda a	$01	3	Load a byte to A at address
lda a, x	$02	3	Load a byte to A at address shifted by X
lda [d], x	$0F	2	Load a byte to A at address located at D
ldx #	$10	2	Load a byte to X
ldx a	$03	3	Load a byte to X at address
sta a	$11	3	Store a value in A to address
sta a, x	$12	3	Store a value in A to address shifted by X
sta [d], x	$1F	1	Store a value in A to address located at D
stx a	$13	3	Store a value in X to address
txx	$14	2	Transfer register 1 value to register 2
and	$04	1	A = A and B
ora	$05	1	A = A or B
xor	$15	1	A = A xor B
lsr	$07	1	A = A >> 1
lsl	$17	1	A = A << 1
add	$08	1	Adds B to A
sub	$18	1	Subtracts B from A
adc	$09	1	Adds Carry to A
sbc	$19	1	Subtract Carry from A
inx	$06	1	Increase X
dex	$16	1	Decrease X
cmp	$1A	1	Compare A to B
jmp a	$0A	3	Jump to address
bne a	$0B	3	Jump to address if Zero flag is clear
beq a	$1B	3	Jump to address if Zero flag is set
bcc a	$0C	3	Jump to address if Carry flag is clear
bcs a	$1C	3	Jump to address if Carry flag is set
bsc a	$0D	3	Jump to address if last branch was successful
bss a	$1D	3	Jump to address if last branch failed
jsr a	$0E	3	Jump to subroutine
rts	$1E	1	Return from subroutine

Also, some commands can change the status register.

• If the command is working with register value (loading, modifying), then Zero flag will be set if result value is zero.
• Some commands can modify Carry flag as overflow. These commands are: lsr, lsl, add, sub, adc, sbc, inx, dex
• The cmp command always modifies Zero and Carry flag (Carry flag is set when A < B).

Indirect Registers

As you know, there is 7 core registers (A, B, X, S, P, I, D). However, with this setup you couldn't do anything but write some values in the RAM or WRM. So, there are a few indirect registers than can be accessed via reading / writing to specific address.

I/O

This register allows you to write and read from console.

 $2000 - I/O Data (r/w)
 
 When written to, displays an ASCII character in console.
 When read, halts until an input, returns a value of pressed key.