z80 » Variables
Once you have the name loaded into OP1 you can
to get the TI86 to return all the information and locations of the variable
whose name is in OP1. Before calling
_FindSym, you need to have OP1
containing the variable name you want to find (case is sensitive) in Variable
Name Format. Once called, it will return: carry flag set if variable by that
name doesn't exist, carry flag reset if variable exists,
variable type, OP1 will still contain the
variable in Variable Name Format,
bde will be an
Absolute Address Pointer to variable data, and
hl will point to the address of the VAT entry of the variable.
|Finds the variable with the name stored in OP1.
|OP1 - case sensitive variable name in
Variable Name Format. Note: You
can leave out the variable type byte,
ignores it and just finds a variable with the same name.
a, bc, de, and
|Carry reset - variable exists
Carry set - variable doesn't exist
a - variable type
OP1 - case sensitive variable name in Variable Name Format
bde - absolute address pointer to variable data
hl - address of the symbol table entry
_FindSym in context...
find_variable: ld hl,variable ;our variable's type and length ; indexed name ld de,OP1 ;where we want it to go ld bc,11 ;OP_'s are 11 bytes long ldir ;(hl)->(de),dec bc,repeat till bc=0 jp _FindSym ;go find it and return variable: .db $0c,$06,"Stupid" ;we don't need any more text ; after "Stupid" because the TI86 ; will just ignore it anyway so we ; just copy whatever is after the ; string along with the type and ; length indexed string
There is a call that will copy 10 bytes starting at
hl into OP1:
_Mov10toOP1. This is even shortened more by just putting
rst 20h instead
call _Mov10toOP1. So the above code would be 22 bytes
(I'm counting opcodes) whereas the
below code, doing exactly the same thing, would only be 13 bytes.
find_variable: ld hl,variable ;our variable's type and length ; indexed name rst 20h ;same as 'call _Mov10toOP1' rst 10h ;same as 'call _FindSym' ret ;done so return variable: .db $0c,$06,"Stupid"
_FindSym ignores the type byte that
the first byte of OP1 is holding. We can put some trash in
there...anything you want. This can be to our advantage because
we can have our routine copy the byte before our 'variable' storage
area as the type; but we need to account for that when we're
hl with the address of the name to be sure
that it's copying starting one byte ahead of this label. This is really
confusing so try to think through our new optimized routine.
find_variable: ld hl,variable-1 ;address of one byte before ; 'variable' label rst 20h ;same as 'call _Mov10toOP1' rst 10h ;same as 'call _FindSym' ret variable: .db $06,"Stupid" ;just need the length and string ; now because we're using the byte ; infront of this as our type
Our final draft of this routine is only 12 bytes long compared to the original that took up 22 bytes. Give me a second to do that higher math...1...4...2...we saved 10 bytes! That may not mean much to you now but in a game, every byte counts...you'll appreciate it later, maybe. (Those byte counts include the variable name which is 8 bytes in the first two examples and 7 bytes in the last example.)
More from z80 » Variables
Absolute Addressing // Binary Coded Decimal // Creating Variables // External Levels // _FindSym // Messing with Variable Data // OP Math // TI-OS Variable Manipulation // Variable Name Format // VAT Searches