The BBC and Master Computer Public Domain Library

8085 Get Address of Variable

Submitted by Steve Fewell

Routine:varaddr
Name: Get Address of Variable
Starting Address: &8085
Entry criteria: &37 and &38 point to the variable name. Y = Variable name length.
Exit: &2A and &2B = the address of the variable block for the variable name [LSB first].
A = &00 if variable not found. &39 = Length of variable Name (Y).

Description:

Store the offset to the end of the variable name in &39 (variable name length).
Load the first character of the variable name. Multiply the ASCII value of the first
character by 2 and store in Y. Y now points to the page &4 offset for the variable pointer
table.
i.e.: If first Char = "A" [ASCII value=&41], then the variable pointer table for
"A" variables is located at &0482.
Note: The resident integer variables (@%, A%, B%, etc...) are handled separately.

Load the MSB of the address stored at the specified variable pointer table location.
If the MSB of the address is zero, then no "A" variables have been defined, so exit with A=&00.

Store the MSB of the variable pointer table address in &2B.
Store the LSB of the variable pointer table address in &2A.

&80A6: Load the 3rd byte of the variable lookup table (&2A, &2B). The 3rd byte is the
beginning of the rest of the variable name [as the variable name is stored without its first
character].

If the variable name character is zero, then we have reached the end of the variable name,
so check whether the length of the variable we are searching for (&39) is the same
as the variable we have found (Y). If not then jump to &8099 to look at the next variable
in the pointer table. If equal, then we have found our variable, so jump to &80C3.
Otherwise (end of name not reached); [&80B6:] Compare the variable name character
(from lookup table) with the next character pointed to by &37, &38.
If the characters don't match then this is not our variable, so goto 8099 to look at next variable
in the pointer table.
Increment Y, and check whether the variable length (&39) has been reached, if not
then (&80B2) load the next character of the variable name in the pointer table (and
jump to &8099 (look at next variable), if the the end of the name was reached (while we still
had more characters in the variable name we were looking for)); otherwise compare this next character [&80B6].
If the end of the variable we are looking for has been reached then check that the next
character of the variable name in the lookup table is zero (indicating the end of the name);
if it isn't, then jump to &8099 to look at the next variable in the lookup table.

Otherwise -> [&80C3:] We have found our variable.
Add length of variable to the address in &2A, &2B -> so that &2A, &2B now
points to the variable details/variable contents section of the variable block (which is located
directly after the variable name). And exit (updating the MSB if any overflow occurred).

[&8099:] Look at next variable in lookup table.
This routine checks byte 2 of the variable block, if it is zero then there is no
next variable address, so exit (with A=00) as the variable doesn't exist.
Otherwise, store the next variable address in &2A (LSB) and &2B and jump to &80A6 to
compare the variable name with the one we are looking for.

Disassembly for the Get Address of Variable routine