Adding the A and B registers in the 8085. Bit 0 is indicated in red, through bit 7 in purple. The B register is the horizontal line in the lower left. The ALU is the block in the upper left.
The bus between the ALU and registers in the 8085. Note that the ALU is stored with bit 0 on the left, while registers are stored with bit 0 on the right.
Location of latches in the 8085. Note the AD pin latches in the upper left corner, ALU in the upper left, registers in the lower left, instruction register in upper right, and miscellaneous other flags and latches.
The 8085 microprocessor, showing the location of the 8-bit ALU.
An XOR gate can be implemented by a NOR gate and an AND-NOR gate. This circuit is used in the 8085 ALU.
Schematic of one bit of the ALU in the 8085 microprocessor
The exclusive-or circuit used in the 8085: gate-level and transistor-level.
The superbuffer used in the 8085 to drive the data bus.
The V flag circuit in the 8085 CPU. This is the silicon/polysilicon after the metal layer has been removed. The data bus is not visible as it is in the metal layer, but it is in the lower third of the image. The rectangles at the bottom connect the data bus to the registers. Chip image from http://visual6502.org.
The 8085 microprocessor chip. The box shows the location of the V flag circuit. Chip image from http://visual6502.org.
The 8085 microprocessor showing the data bus, ALU, flag logic, registers, and incrementer/decrementer. Chip image from http://visual6502.org.
The metal layer of the 8085 microprocessor, zoomed in on the V flag circuit. Chip image from http://visual6502.org.
Structure of a MOSFET transistor. The n+ diffusion regions are pink, the polysilicon gate conductor is dull green, and the insulating oxide layer is turquoise.
A NOR gate is implemented through two transistors and a pullup transistor. If either (or both) input is 1, the corresponding transistor connects the output to ground. Otherwise, the transistors are open, and the pullup pulls the output high.
A MOS transistor functions as a simple switch that closes when 1 is applied to the gate.
A NOR gate in the 8085 microprocessor, showing the components.If either input is high, the associated transistor will connect the output to ground. Otherwise the pullup transistor will pull the output high.
An XOR gate in the 8085 microprocessor, formed from a NOR gate and an AND-NOR gate. If both inputs are 0, the NOR gate output will be 1, and the NOR transistor will pull the output to 0. If both inputs are 1, the AND transistors will pull the output to 0. Otherwise the pullup transistor will pull the output 1.
Each flag in the 8085 uses a superbuffer to drive the value onto the data bus. This figure shows the two large transistors that drive the V flag onto bit 1 of the data bus.
Each flag in the 8085 uses a two-inverter latch to store the flag. This shows the latch for the undocumented V flag. The red arrows show the flow of data.
The circuit in the 8085 for the undocumented V and K flags. The flags are generated from the carries and results from the ALU. The K flag can also be set by the carry from the incrementer/decrementer.
The latch used in the 8085 to store a flag value. The latch uses two inverters to store the data. When the clock is low, a new value can be written to the latch.
The 8085 circuits to implement the undocumented V and K flags. The ALU provides /carry6, /carry7, and result7. The XOR circuit on the left generates V, and the XOR circuit in the middle generates K. On the right are the latch for the V flag, and the superbuffer that outputs the flag to the data bus. The K flag latch and superbuffer are to the right, not shown.
Photograph of the 8085 chip showing components relevant to register operations.
Schematic of the 8085's amplifier to read one bit of the register file into the address latch.
Schematic of the amplifier/driver to read one bit of the register file to the data bus.
Schematic of the 8085 circuit to select the three register select bits.
Schematic of the 8085 circuit to select the DE or HL registers. The flip flop in at the bottom switches the role of DE and HL in response to an XCHG instruction.
Detail of the 8085 chip showing six bits in the 8085's register file. Bit 2 of the stack pointer is shown with schematic. The two transistors form two inverters in a feedback loop. The light blue lines are the metal layer wires connected to bit 2. The program counter is in the upper half of the image.
Architecture diagram of the 8085 register file, as it is implemented on the chip. The register file is connected to the data bus at top, and address bus at bottom. The control lines are along the right.
Two bits of the 8085 register file. Each bit is stored in two inverters in a feedback loop. The register bus uses two lines of opposite polarity for each bit.
One bit of a register in the 8085 register file. Each bit is stored in two inverters in a feedback loop. The register bus uses two lines of opposite polarity for each bit. Access to the register is controlled by the reg_rw control line, which connects the inverters to the bus, allowing the value to be read or written.
A closeup of the 8085 microprocessor, showing the details of the register file and the locations of the major components.
Two bits of the 16-bit increment/decrement circuit in the 8085. Odd bits and even bits use a different circuit for efficiency. The carry out from even bits is complemented.
Schematic of the carry circuitry in the 8085 microprocessor.
Schematic of the decimal adjust circuitry in the 8085 microprocessor.
Architecture of the 8085 ALU according to the datasheet. This does not quite match the actual architecture.
Architecture of the 8085 ALU as determined from reverse-engineering.
Schematic of the flag storage in the 8085 microprocessor.
Photograph of the 8085 chip showing the location of the ALU, flags, and registers.
The ACT (Accumulator Temporary) register in the 8085 provides one of the inputs to the ALU. The value loaded into the register is selected by several control lines.
The ACT register in the 8085. This image shows the silicon that implements the 8-bit register. Each of the large pink structures is one bit. Bit 7 is on the left and bit 0 on the right.
The accumulator and ACT (Accumulator Temporary) registers and their control lines in the 8085 microprocessor.
The 8-bit ALU in the 8085 is formed by combining eight 1-bit slices.
The silicon that implements the flags in the 8085 microprocessor.
One bit of the ALU in the 8085 microprocessor.