MSaarna at 17:10, 26 September 2018

2018-09-26T17:10:01Z

MSaarna: Created page with "==jump table instead of comparison branch chains== When you need to choose between 4 or more destinations, it's more efficient to use a jump table instead of a sequence of CM..."

2015-12-22T18:52:07Z

Created page with "==jump table instead of comparison branch chains== When you need to choose between 4 or more destinations, it's more efficient to use a jump table instead of a sequence of CM..."

New page

==jump table instead of comparison branch chains==

When you need to choose between 4 or more destinations, it's more efficient to use a jump table instead of a sequence of CMP and BEQ. Here's a jump table example that uses RTS instead of JMP (ADDRESS) to save even more ROM and cycles...

LDX FunctionIndex
LDA FunctionJumptablehi,x
PHA
LDA FunctionJumptablelo,x
PHA
RTS
FunctionJumptablehi
.byte >(ClearRam-1)
.byte >(SetRam-1)
.byte >(MemCpy-1)
.byte >(MemCmp-1)
FunctionJumptablelo
.byte <(ClearRam-1)
.byte <(SetRam-1)
.byte <(MemCpy-1)
.byte <(MemCmp-1)

==fast range test==

For all of these we assume that the byte to be tested is in A and that the start and end values, n and m, are already defined. Also that 0 < n < m < $FF.

If you don't need to preserve the byte in A then testing the byte can be done in five bytes and only six cycles. This sets the carry if A is in the range n to m.

CLC ; clear carry for add
ADC #$FF-m ; make m = $FF
ADC #m-n+1 ; carry set if in range n to m

Courtesy Lee Davidson. <ref>[http://web.archive.org/web/20130212081001/http://mycorner.no-ip.org/6502/shorts/range.html Lee Davidson's 6502 Shorts]</ref>

==References==

6502 Advanced Tips - Revision history

MSaarna at 17:10, 26 September 2018

MSaarna: Created page with "==jump table instead of comparison branch chains== When you need to choose between 4 or more destinations, it's more efficient to use a jump table instead of a sequence of CM..."