Jumps

There are two kinds of jump instructions. Absolute and relative.

Absolute jumps

JP cc,address
Jump to address if condition code cc is true. Otherwise advance the program counter to the next instruction.
Address is a direct 16 bits address. You can jump to any address from 0 - 64k

Relative jumps

JR cc,relative_address
Add relative address to programcounter if condition code cc is true. Otherwise advance the program counter to the next instruction.>
Address is a relative 8 bits address. You can only jump between -128 or +127.
Note: You can use JR with labels like this: JR my_label since the assembler will take care of calculating the relative address.

Condition Codes

If you always want to jump no matter what you don't have to specify a condition code, but if you only want to jump when a certain set of contitions are met you need to add one of these codes.
Note: You can only have one condition code per jump instruction.

HexMnemonicDefinitionFlag Test Operation
0 F Always False -
1 LT Less Than (S XOR V)=1
2 LE Less Than or Equal (Z OR (S XOR V)) = 1
3 ULE Unsigned Less Than or Equal (C OR Z) = 1
4 OV Overflow V=1
5 MI Minus S=1
6 Z Zero Z=1
6 EQ Equal Z=1
7 C Carry C=1
7 ULT Unsigned Less Than C=1
8 T(or blank) Always True(default) -
9 GE Greater Than or Equal (S XOR V) = 0
A GT Greater Than (Z OR (S XOR V)) = 0
B UGT Unsigned Greater Than (C = 0 AND Z = 0) = 1
C NOV No Overflow V=0
D PL Plus S=0
E NZ Non-Zero Z=0
E NE Not-Equal Z=0
F NC No-Carry C=0
F UGE Unsigned Greater Than or Equal C=0

Example code

Jump to a label
main:		
	;This code will loop infinitely.
	jp main
		
Jump relative to a label
main:
	;This code does the same as the previous code: loop indefinitely.
	jr main
		
Jump relative to anonymous label
	
$$:	
	jr $F ;Jump Forward to next $$ label (anonymous label)
	jr $B ;Jump Back to previous $$ label (anonymous label)
$$:
		
Jump to local label
	
$loop: jp $loop ; Infinite branch to $loop
lab?: jp lab?   ; Infinite branch to lab?
scope           ; New local label scope
$loop: jp $loop ; Reuse $loop
lab?: jp lab?   ; Reuse lab?
		

Notice that you have to use extended addressing to be able to access these registers. They are not in range of standard 8 bit addressing.


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