RCA CDP18S711 manual Machine Language Programming Summed Up

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The CHIP-8 language described in the previous section, permits machine language subroutines to be called with a 0MMM instruction. A D4 machine language instruction at the end of the machine language subroutine returns control to the CHIP-8 instruction following the 0MMM instruction. In Appendix C, the CDP1802 register use for the CHIP8 language is provided. R5 is used as the CHIP-8 program counter. When you call a machine language subroutine with a 0MMM instruction, R5 will be addressing the CHIP-8 instruction following the 0MMM. The machine language subroutine could retrieve the next two CHIP-8 program bytes as parameters by addressing with R5 and incrementing it by 2 before returning control to the CHIP-8 program with a D4 instruction. RC, RD, RE, and RF are available for use in machine language subroutines. RA is the CHIP-8 memory pointer (1). Changing the high-order byte of RB will cause any desired RAM page to be displayed. R3 is the machine language subroutine program counter.

CHIP-8 uses the operating system refresh interrupt routine contained in ROM for display. You can use this ROM interrupt routine for 256-byte display in your own machine language programs. First initialize R1 to 8146 and R2 as a stack pointer before turning on the video interface with a 69 instruction. Set the desired display page into RB.1. This interrupt routine uses R0 as the display refresh pointer and modifies RB.0. R8.1 and R8.0 are decremented by 1 during each interrupt unless they are equal to 00. Interrupts occur 60 times per second when the video interface is turned on. This rate is controlled by a crystal clock so that R8.0 and R8.1 can be used as real-time clocks when needed. -

RCA COSMAC VIP Instruction Manual

While the video interface is turned on, you should not use any of the 3-machine-cycle CDP1802 instructions (except those used for sync in the refresh interrupt routine itself). If you are not using the video interface, then you can use the CDP1802 3-cycle instructions in your machine language programs. When you initiate a machine language program at 0000 by flipping RUN up, the video interface will be off. You must turn it on with a 69 instruction to use the COSMAC VIP graphic display capability.

Machine Language Programming Summed Up

In summary, COSMAC VIP provides you with an easy-to-use language called CHIP-8. You can insert machine language subroutines in CHIP-8 programs for greater flexibility or expanded I/0 capability. You can write complete machine language programs to fully utilize CDP1802 capabilities. The operating svstem facilitates debugging machine language programs by permitting you to examine general registers R3-RF. (See operating system register table in Appendix B). Advanced programmers can even develop their own, interpretive language tailored to special requirements. Direct execution of machine language code starting at location 0000 together with the expansion interface permits the COSMAC VIP system to be used as a low-cost development system as well as a personal recreational or educational computer.