A51: Lookup Tables in Assembly

Information in this article applies to:

• C51 All Versions

QUESTION

In assembly, I must create a lookup table and locate it at a fixed address. I will access the table in an assembly function that I must call from C. How should I proceed?

ANSWER

The example project attached to this knowledgebase article contains three source files:

• TEST.C: Contains the C program which calls the assembler function.
• ASM_TEST.ASM: Contains the assembler function which uses the lookup table.
• LOOKUPTABLE.INC: Contains the lookup table.

The contents of each file is listed below.

To locate the lookup table at a fixed address of 0x1000, open Project - Options for Target - BL51 Locate. In the Code Segment field enter SEGMENT_NAME(start_address). For this example, you should enter LOOKUPTABLE(0x1000). For your application, you may need to look up the segment name from the map file (*.M51).

LOOKUPTABLE.INC

LOOKUPTABLE SEGMENT CODE

RSEG LOOKUPTABLE

TABLE:   DB 02H, 03H, 04H, 05H, 06H, 07H, 08H, 09H

ASM_TEST.ASM

NAME ASMTEST

$include(LOOKUPTABLE.INC)

?PR?TSEG?ASM_TEST SEGMENT CODE
PUBLIC  ASM_LOOKUP_ROUTINE

RSEG ?PR?TSEG?ASM_TEST
using   0

ASM_LOOKUP_ROUTINE:

   MOV DPTR, #TABLE    ; DPTR points to the start of the lookup table
   MOV A, #4           ; A is the offset from the start of the lookup table
   MOVC A, @A + DPTR   ; Moves the (A+1)th item into the Accumulator

   RET
   END

TEST.C

extern void ASM_LOOKUP_ROUTINE(void);

void main(void)
{
   ASM_LOOKUP_ROUTINE();     // calling lookup routine

   while(1);
}

SEE ALSO

• GENERAL: String Tables in C
• µVision: Including C SRC Files in a Project

ATTACHED FILES

Request the files attached to this knowledgebase article.

Last Reviewed: Thursday, February 25, 2021