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The CPU used in the Intellivision is a General Instruments CP1610. The CP1610 is a general purpose microprocessor capable of supporting 16-bit addresses and 10-bit instructions.
|Clock Speed||894,886.25 Hz (NTSC)|
|Instruction Opcode Width||10-bit|
|Flags||S, Z, O, C, I, D|
|Registers||Eight 16-bit Registers, R0-R7|
The R7 register behaves as program counter for the CP1610. On the Intellivision, the CP1610 initializes R7 to the value $1000. The CP1610 then reads, decodes, and executes the opcode at the location reference by R7 and the increments R7 to the next instruction. Although most opcodes are only one memory location in length (one 10-bit value or "decle"), a few opcodes stretch to two or three memory locations, causing R7 to increment by more than a single value in order to find the next instruction to execute. The CP1610 repeats this behavior indefinitely, or until a HLT instruction is read.
The processor also receives hardware interrupts from the STIC which occur once per screen refresh and cause the CP1610 to jump to the interrupt subroutine located at $1004 in the Executive ROM. The STIC may also request the CP1610 temporarily halt processing to allow the STIC to perform direct memory accesses.
|S||Sign Flag||If set, indicates that the previous operation resulted in negative value, which is determined by a one (1) in bit 15 of the result.|
|C||Carry Flag||If set, indicates that the previous operation resulted in unsigned integer overflow.|
|Z||Zero Flag||If set, indicates that the previous operation resulted in zero.|
|O||Overflow Flag||If set, indicates that the previous operation gave a signed result that is inconsistent with the signs of the source operands. (Signed overflow.)|
|I||Interrupt Enable Flag||If set, allows the INTRM line to trigger an interrupt, causing the CPU to jump to the interrupt subroutine located in the Executive ROM at $1004.|
|D||Double Byte Data Flag||If set, it causes the next instruction to read a 16-bit operand with two 8-bit memory accesses, if it supports it.|
- The S, C, Z, and O flags are directly visible to the programmer via the GSWD instruction. The I and D flags are internal to the CPU, and not visible directly visible to the programmer.
- The C and O flags serve a secondary purpose for the shift and rotate instructions. The SLLC, SARC, RLC and RRC use the C and O flags to store bits that get "shifted away." Also, the RLC and RRC instructions "shift in" the bits stored in C and O.
- The S flag takes on a special role also with the shift instructions. The SWAP, SLR, SAR, SARC and RRC instructions set the S flag based on bit 7 of the result, rather than bit 15.
|Signal||Name||Direction||Purpose||Use on Intellivision|
|INTRM||INTerrupt Request, Masked.||Input||A high-to-low transition on this pin causes the CPU to take an interrupt if interrupts are enabled. If the signal transitions back to the "high" state prior to taking the interrupt, the CPU will ignore the interrupt.||Connects to the SR1 output pin on the STIC. The STIC signals VBlank Interrupt to the CP1610 on this pin, and the CP1610 jumps to the interrupt vector at $1004 after the next interruptible instruction.|
|INTR||INTerrupt Request.||Input||A high-to-low transition on this pin causes the CPU to take an interrupt, regardles of whether interrupts are enabled.||The Intellivision leaves this signal tied to +5v (deasserted).|
|BUSRQ||BUS ReQuest.||Input||A high-to-low transition on this pin requests that the CPU halt so that something else may access the bus.||Connects to the SR2 output pin on the STIC. The STIC signals the CP1610 on this pin at points during active display when it needs to access System RAM. The CP1610 halts after the next interruptible instruction, regardless of whether interrupts are enabled.|
|BUSAK||BUS AcKnowledge.||Output||Asserted (active-low output) when the CPU has yielded the bus.||Connects to the SST input pins on the System RAM and the GROM. The CP1610 acknowledges the STIC's bus request by signaling to the System RAM and GROM that it halted by request of the STIC.|
|MSYNC||Master SYNC.||Input||Asserting this signal (active low) resets the CPU and synchronizes it to its clocks.||Connects to the MSYNC output of the STIC. The STIC generates this signal shortly after powerup after the clock stabilizes, or whenever someone releases the reset button or asserts RESET on the cartridge port.|
|EBCA0 - EBCA3||External Branch Condition Address.||Output||The four outputs EBCA0 through EBCA3, along with the single input ECBI provide a mechanism for external hardware to generate branch control inputs to the CPU. The BEXT instruction includes a 4-bit field that the CP1610 asserts on EBCA0 - EBCA3. External hardware then asserts a 0 or 1 on EBCI to indicate whether to take the branch. This allows up to 16 different branch conditions to be asserted to the CPU.||These pins are not connected in the Intellivision, so the BEXT instruction is typically never used.|
|EBCI||External Branch Condition Input.||Input|
|TCI||Terminate Current Interrupt.||Output||TCI is both an instruction as well as a pin on the device. The TCI instruction pulses the TCI pin on the CP1610.||The Intellivision leaves this signal unconnected, and so the TCI instruction is effectively a NOP.|
|PCIT||Program Counter Inhibit/Trap.||Bidirectional.||External hardware can prevent the CPU from incrementing the program counter by asserting this signal. The SIN instruction generates a pulse on this line.||The Intellivision leaves this signal tied to +5v (deasserted) through a resistor.|
|STPST||SToP-STart.||Input||Stops or starts the execution of the CPU whenever it sees a high-to-low transition. Can restart the CPU after a HLT instruction.||The Intellivision leaves this signal tied to +5v.|
|HALT||HALTed.||Output||Indicates the CPU halted, either due to STPST or a HLT instruction.||The Intellivision leaves this signal unconnected.|
|BDRDY||Bus Data ReaDY.||Input||When deasserted, it causes the CPU to wait for data to become available on the bus, effectively inserting wait states.||The Intellivision leaves this signal unconnected, and thus has no notion of wait states.|
Each CP1610 opcode is considered either "interruptible" or "not interruptible". The interruptibility of an opcode determines whether or not the CP1610 checks the status of INTRM and BUSRQ after executing that instruction, and prior to executing the next. The CP1610 will check for an interrupt request signaled via INTRM only if both the Interrupt Enable Flag flag is set and previous instruction was an interruptible instruction. It is important to understand that the Interrupt Enable Flag and the interruptibility of each opcode are completely separate functions and both work together to control when the CP1610 accepts interrupts on the INTRM line. For more information on interrupts, see the VBlank Interrupt topic.
Similarly, the STIC only asserts BUSRQ to the CP1610 for 114 CP1610 clock cycles (on NTSC systems). The Interrupt Enable Flag does not affect the CPU's ability to respond to this bus request. Non-interruptible instructions, however, do. Thus, if a program contains an extended sequence of non-interruptible instructions, the CPU will not halt in time for the System RAM to prepare the next row of display cards. Instead, the machine will duplicate the previous row and push the rest of the display down. It appears that the CPU must halt within about 57 cycles of
BUSRQ first being asserted in order to prevent display glitches.
There are eight 16-bit registers available in the CP1610, labelled R0-R7.
|R4||General Purpose. Auto-increments on indirect reads and writes.|
|R5||General Purpose. Auto-increments on indirect reads and writes.|
|R6||SP||Stack Pointer. Auto-increments on indirect reads. Auto-decrements on indirect writes.|
|R7||PC||Program Counter. Auto-increments on indirect reads and writes.|
The names "SP" and "PC" are assembler aliases for "R6" and "R7," respectively. They may be used interchangeably.
The CP1610 supports several addressing modes. Click on any of the addressing modes below for more information.
|Implied||The opcode's inputs or outputs are implied by the opcode itself.|
|Register||The value to be read or written is contained in one or more registers. No memory access are required to execute Register-mode opcodes.|
|Direct||The address of the value to be read or written is specified by the value immediately following the address of the opcode.|
|Immediate||The address of the value to be read or written is the address immediately following the opcode. Note that immediate mode addressing is really just Indirect addressing using the R7 register.|
|Indirect||The address of the value to be read or written is contained in one of the registers.|
|Stack||This Indirect Mode through R6, the stack pointer.|
Below is a detailed breakdown of all the opcodes available as a part of the CP1610 instruction set. Click on each opcode for more information. Also, you might look at the original GI Instruction Set Reference, which is a handy (if occasionally incomplete or inaccurate) guide.
|Range||Instruction||Mnemonic(s)||Cycles||Interruptible||Input Flags||Output Flags|
|$0001||Set Double Byte Data||SDBD||4||No||D|
|$0002||Enable Interrupt System||EIS||4||No||I|
|$0003||Disable Interrupt System||DIS||4||No||I|
|$0005||Terminate Current Interrupt||TCI||4||No|
|$0028-$002F||Add Carry to Register||ADCR||6||Yes||C||S||Z||O||C|
|$0030-$0033||Get the Status Word||GSWD||6||Yes||S||Z||O||C|
|$0038-$003F||Return Status Word||RSWD||6||Yes||S||Z||O||C|
|$0048-$004F||Shift Logical Left||SLL||6/8||No||S||Z|
|$0050-$0057||Rotate Left through Carry||RLC||6/8||No||O||C||S||Z||O||C|
|$0058-$005F||Shift Logical Left through Carry||SLLC||6/8||No||O||C||S||Z||O||C|
|$0060-$0067||Shift Logical Right||SLR||6/8||No||S||Z|
|$0068-$006F||Shift Arithmetic Right||SAR||6/8||No||S||Z|
|$0070-$0077||Rotate Right through Carry||RRC||6/8||No||O||C||S||Z||O||C|
|$0078-$007F||Shift Arithmetic Right through Carry||SARC||6/8||No||O||C||S||Z||O||C|
|$0248-$026F||Move Out Indirect||MVO@||9||No||D|
|$0270-$027F||Move Out Immediate||MVOI||9||No||D|
|$0288-$02AF||Move In Indirect||MVI@||8/10/11||Yes||D|
|$02B0-$02BF||Move In Immediate||MVII||8/10||Yes||D|