Look-up Tables and Jump Tables

Look-up tables (LUTs)

To make a look-up table (LUT), you must set the bounds. Look-up tables are often used in calculations which are not easy to find with arithmetic, such as the sine function. Lets model the sine function with a look-up table.

    LD     H, 0
    LD     L, A
    LD     DE, sine_table
    ADD    HL, DE
    LD     A, (HL)
    INC    HL
    LD     H, (HL)
    LD     L, A
    ; The lookup table
.DW    $0000, $0004, $0009, $000D, $0012, $0016, $001B, $001F, $0024
.DW    $0028, $002C, $0031, $0035, $003A, $003E, $0042, $0047, $004B
.DW    $004F, $0053, $0058, $005C, $0060, $0064, $0068, $006C, $0070
.DW    $0074, $0078, $007C, $0080, $0084, $0088, $008B, $008F, $0093
.DW    $0096, $009A, $009E, $00A1, $00A5, $00A8, $00AB, $00AF, $00B2
.DW    $00B5, $00B8, $00BB, $00BE, $00C1, $00C4, $00C7, $00CA, $00CC
.DW    $00CF, $00D2, $00D4, $00D7, $00D9, $00DB, $00DE, $00E0, $00E2
.DW    $00E4, $00E6, $00E8, $00EA, $00EC, $00ED, $00EF, $00F1, $00F2
.DW    $00F3, $00F5, $00F6, $00F7, $00F8, $00F9, $00FA, $00FB, $00FC
.DW    $00FD, $00FE, $00FE, $00FF, $00FF, $00FF, $0100, $0100, $0100

The main disadvantage is the size, but we can't do much about it.

Jump Tables

These are just like LUTs, except that rather than values being returned, they return an area in memory.

A vector table holds only the addresses.
Here is an example:

    .DW     ClearScreen
    .DW     PutSprite
    .DW     DrawLine
    .DW     EndPrgm
    .DW     BlackScreen

    LD     H, 0
    LD     L, A
    ADD    HL, HL
    LD     DE, VectTbl
    ADD    HL, DE;Just like a look-up table

    LD     A, (HL)
    INC    HL
    LD     H, (HL)
    LD     L, A
    JP     (HL)

A Jump table is essentially a vector table, but rather than holding the address, it holds the entire jump instruction.

    JP    ClearScreen
    JP    PutSprite
    JP    DrawLine
    JP    EndPrgm
    JP    BlackScreen

To call or jump to a routine in the jump table, you use an address of
JumpTbl + 3 * n
where n is the number of the routine you want. Supposing you wanted to run DrawLine, then you would use
CALL JumpTbl + 3 * 2

If you notice, the default B_CALLs have offsets of 3. For example, B_CALL(_ClrLCDFull) is EF4045 in machine code, and B_CALL(_ClrLCD) is EF4345 in machine code, and B_CALL(_ClrScreenFull) is EF4645. EF is BCALL(**). But why is the first byte of the address increasing? This is because the calculators use Little-Endian; the least-significant-byte and most-significant-byte are essentially swapped.

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