mjhjmtu

Right now I am learning 6502 assembly. Currently I am using the MADS assembler to program for the Atari 800. This program is just a small tutorial program that came with the assembler zip file I downloaded. The only piece of this program I did't understand is the jmp * operation, what does the '*' mean? Full program:

sm_ptr = $58
ch = $2f4 
chr = $2400 
rom_chr = $e000 
org $2000
.proc main 
mva #>chr ch 
ldx #0 
copy_loop
mva rom_chr,x chr,x
mva rom_chr+$100,x chr+$100,x
mva rom_chr+$200,x chr+$200,x
mva rom_chr+$300,x chr+$300,x
inx
bne copy_loop
ldx #0 
space_loop
mva charset.space,x chr,x
inx
cpx #8
bne space_loop 
ldy #0 
loop 
tya
sta (sm_ptr),y
iny
bne loop
jmp *
.local charset
space 
.byte %01000100 
.byte %00101000
.byte %01111100
.byte %01010100
.byte %11111110
.byte %10101010
.byte %10111010
.byte %00000000
.endl
.endp 
run main

MADS uses * in three ways (See MADS "Manual")

• Using the current assembly address for calculation of an address, i.e. the one the actual statement is assembled to.


• Multiplying in expressions.


• Mark the beginning of a comment (until line end)

In above listing it will be interpreted as the address the JMP instruction is assembled to, so it will form an infinite loop, effectively halting the machine until Reset.

I suspect the * means the current instruction/location, as it does in some other assemblers, like PDP-8.  Often it would be used in an expression like *-label in the data section to get something's size, or *+3 perhaps, in code.

If that's the case for 6502 assembly, then jmp * means branch to self, or, infinite loop, which would be a form of halting the program.

This makes some sense as it comes at the end of main, which presumably has nothing to return to for a bare metal program.

Some assemblers (e.g. masm, Microsoft's x86 assembler) use $ for the same meaning.