I'm currently learning x86/x64 asm and I wanted to try to make a jump table, but I can't figure out what I'm doing wrong.
The concept itself is not new to me, I just can't figure out why it doesn't work. I saw the usage of [ ] in a few times when I was researching this, but I'm not sure if it is the right way to do it.
.data
var qword 10
.code
main proc
mov rax, var
jmp [table]
back:
ret
table:
qword subroutine, subroutine2
subroutine:
mul var
jmp back
subroutine2:
mul var
jmp back
main endp
end
When I step through the code, and it skips the jmp instruction and on ret gives an access violation reading location 0x00000000
error
Note that as commented, MASM is ignores the []. Instead MASM goes by the type for a label. In this case, the problem is that the : after table (table:) makes label of type "code" that is normally used as a branch or call target, so jmp [table] or jmp table, branches to table as if it were code.
Removing the : and putting the qword (or dq could be used) on the same line, changes table to type qword, so jmp [table] or jmp table, loads the qword address at table into RIP and does the branch as wanted.
table qword subroutine, subroutine2
However, if you want to index into table, you'll either need to use a register to hold the offset of the table (like lea r9,table), or in the case of Visual Studio, go to project / properties / linker / system / enable large addresses : no (sets linker parameter /LARGEADDRESSAWARE:NO). I posted examples for both cases below.
This example works with ML64.EXE (MASM) from Visual Studio. The table can be in code or data section. If table is first line in data, lea generates {4C 8D 0D 79 E5 00 00}, if table is first line in code, lea geneates {4C 8D 0D E1 FF FF FF}. I don't know which is better for performance. It would seem that if the data cache is not being fully utilized, then it would keep a copy of the table the data cache.
.data
tbl dq fun1, fun2, fun3 ;table
.code
main proc
lea r9,tbl
mov rax,0
main0: jmp qword ptr [r9+rax*8]
main1:: inc rax
cmp rax,3
jb main0
xor eax,eax
ret
main endp
fun1 proc
mov rdx,1
jmp main1
fun1 endp
fun2 proc
mov rdx,2
jmp main1
fun2 endp
fun3 proc
mov rdx,3
jmp main1
fun3 endp
end
With Visual Studio linker parameter /LARGEADDRESSAWARE:NO, there no need to use a second register. The table can be in data or code section. If table is first line in data, jmp generates {FF 24 C5 00 00 3D 00}, if table is first line in code, jmp geneates {FF 24 C5 80 1A 2D 01}. I don't know which is better for performance. It would seem that if the data cache is not being fully utilized, then it would keep a copy of the table the data cache.
.data
tbl dq fun1, fun2, fun3 ;table
.code
main proc
mov rax,0
main0: jmp qword ptr [tbl+rax*8]
main1:: inc rax
cmp rax,3
jb main0
xor eax,eax
ret
main endp
fun1 proc
mov rdx,1
jmp main1
fun1 endp
fun2 proc
mov rdx,2
jmp main1
fun2 endp
fun3 proc
mov rdx,3
jmp main1
fun3 endp
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