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Besides user defined symbols, MIXAL programmers can use the so called
local symbols, which are symbols of the form [1-9][HBF]
. A
local symbol nB
refers to the address of the last previous
occurrence of nH
as a label, while nF
refers to the next
nH
occurrence. Unlike user defined symbols, nH
can appear
multiple times in the LABEL
part of different MIXAL
instructions. The following code shows an instance of local symbols’
usage:
* line 1 1H LDA 100 * line 2: 1B refers to address of line 1, 3F refers to address of line 4 STA 3F,2(1B//2) * line 3: redefinition of 1H 1H STZ * line 4: 1B refers to address of line 3 3H JMP 1B
Note that a B
local symbol never refers to a definition in its
own line, that is, in the following program:
ORIG 1999 ST NOP 3H EQU 69 3H ENTA 3B local symbol 3B refers to 3H in previous line HLT END ST
the contents of ‘rA’ is set to 69 and not to 2001. An
specially tricky case occurs when using local symbols in conjunction
with ORIG
pseudoinstructions. To wit9,
ORIG 1999 ST NOP 3H CON 10 ENT1 * LDA 3B ** rI1 is 2001, rA is 10. So far so good! 3H ORIG 3B+1000 ** at this point 3H equals 2003 ** and the location counter equals 3000. ENT2 * LDX 3B ** rI2 contains 3000, rX contains 2003. HLT END ST