From Intellivision Wiki
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 (??).