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System diagram

A project log for Kobold - retro TTL computer

A computer with 20 address bits, 16 bit instructions, video display, from just a few TTL and memory chips. Aiming for PDP11 instruction set.

roelhroelh 04/21/2019 at 09:130 Comments

This is the new system diagram for the 16 bit bus system.

The program counter is in the set of four registers. However, incrementing the program counter would either cost several extra cycles or extra hardware. In this low-part-count solution, the lowest 4 bits of the PC are in a hardware counter, and the other bits are provided by the register set. When the 4-bit counter overflows, the microcode will add 16 to the part that is in the register set.

Register B is a temporary register for MOV instructions. It is loaded with 16 bit data from a source memory location, and then its contents can be stored to a destination memory location. These locations include the R0-R7 because they are also in memory. Register B can also shift data. It is constructed of two 74HC299 devices.

The ALU can do only two instructions, ADD and a logical instruction, NOR or NAND, [TBD, to be determined]. Other instructions are done by microcode. The ALU takes one operand from one of the registers PC, WP, X or Y, and the other operand from A. The outputs of  these registers can be tri-stated in 8-bit sections, so adding A to X will first enable the low section of X and A, add them and put the result in the low section of R. Then the same will be done for the high byte. 

Note that the ALU can also swap the two bytes in a word. This will be used when byte instructions need to store a byte in the upper 8 bits of a memory word. The microcode will look at address bit 0 to determine if a byte must be moved to or from the low or high part of a memory word. For storing a byte to a memory location, the microcode can disable writing to one of the two bytes that  are in a memory word, so when a byte is written, the other byte in that memory word is not affected.

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