Executive ROM

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The Executive ROM is 5KB of ROM arranged as 4096 10-bit values, referred to as "decles", mapped into the Intellivision address range $1000-$1FFF. On the Intellivision II only, the Executive ROM includes an additional 256 decles at addresses $0400-$04FF. On the Intellivision I, two chips hold the EXEC ROM: an RO-3-9502 and an RO-3-9504. On the Intellivision II, these two ROMs were combined into a single RO-3-9506.

The Executive ROM provides the "operating system" for the Intellivision. When the Intellivision is turned on or reset, the CP1610 begins executing instructions from the Executive ROM starting at location $1000. Additionally, when the STIC generates its VBlank Interrupt, the CP1610 jumps to the interrupt-handling subroutine located in the Executive ROM at location $1004. The RO-3-9502 (or RO-3-9506) chip asserts these addresses to the CPU during the IAB Bus Phase. Cartridges can theoretically override these addresses by intercepting the IAB bus phase, asserting the desired address, and remapping IAB to NACT.

Cartridge ROM usually originates at address $5000 and, after completing its startup sequence, the Executive ROM jumps to the start of the program contained in the cartridge at that location. However, prior to entering the startup sequence, the Executive ROM first checks addresses $4800 and $7000 for viable ROM, and if it exists, the Executive ROM skips the normal startup process and begins running the program immediately. This is why some games start their map at $4800, to nearly completely bypass the Executive ROM start-up sequence.

Known Executive ROM Subroutines

 $1668  Extend sign from low byte of R0 into full word.
 $1DDC  Multiply: R0 and R1.  Result in R2.
 $1DFB  Divide:  Dividend is R1, divisor is R2.
        Quotient placed in R0, remainder placed in R1.
 $1738  Zeros memory.
        R0 = count
        R4 = address
 $1741  Fills memory with value in R1.
        R0 = count
        R1 = value
        R4 = address
 $1777  R1 = start addr.
        Returns:
        R2 = x.pos
        R3 = y.pos
 $18C5  Write a numeric string, padded with spaces on the left instead of
        0 digits.  R4 is positioned at the begining of where the numeric
        was written to, instead of positioned just after.
        R0 = numeric value
        R1 = number of digits
        R3 = attribute word (same as BACKTAB word format)
        R4 = destination address to write to (i.e. BACKTAB location)
 $18AD  Write a numeric string like $18C5, but pad with 0's.
        R0 = numeric value
        R1 = number of digits
        R3 = attribute word (same as BACKTAB word format)
        R4 = destination address to write to (i.e. BACKTAB location)
 $1867  Same as $187B, except you can specify an address from where to
        fetch the string data.  The address is incremented to point to
        the character after the null terminator, but is placed in R5.
        R1 = address of string data, which must end with a null
             terminating character ($0).
        R3 = attribute word (same as BACKTAB word format)
        R4 = destination address to write to (i.e. BACKTAB location)
 $187B  Write a string of words to a memory location (such as BACKTAB).
        The string of words must follow the JSR instruction, and must
        end with a null-terminated word (0h).  The R7 register will be
        automatically incremented to start running the instruction
        right after the null-terminated word.
        R3 = attribute word (same as BACKTAB word format)
        R4 = destination address to write to (i.e. BACKTAB location)
 $1B95  Possibly triggers a "stock" sound effect.
        Parameter is passed as a string of words after the JSR instruction,
        and is terminated by a null-terminated word. The R7 register will
        be automatically incremented to start running the instruction
        right after the null-terminated word.
 $1BBE  Possibly generates a noise envelope.  The data is passed after the
        JSR instruction, and the R7 register is automatically incremented
        to start running the instruction right after a null-terminated word
        of $02CF (??).
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