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MAR2901FC Datasheet(PDF) 4 Page - Zarlink Semiconductor Inc |
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MAR2901FC Datasheet(HTML) 4 Page - Zarlink Semiconductor Inc |
4 / 13 page MA2901 3 This multiplexer scheme gives the capability of selecting various pairs of the A, B, D, Q and “0” inputs as source operands to the ALU. These five inputs, when taken two at a time, result in ten possible combinations of source operand pairs. These combinations include AB, AD, AQ, A0, BD, BQ, B0, DQ, D0 and Q0. It is apparent the AD, AQ and A0 are somewhat redundant with BD, BQ and B0 in that if the A address and B address are the same, the identical function results. Thus, there are only seven completely non-redundant sourced operand pairs for the ALU. The MA2901 microprocessor implements eight of these pairs. The microinstruction inputs used to select the ALU source operands are the l0, I1, and I2 inputs. The definition of l0, I1, and I2 for the eight source operand combinations are as shown in figure 2. Also shown is the octal code for each selection. The two source operands not fully described as yet are the D input and Q input. The D input is the four-bit wide direct data field input. This port is used to insert all data into the working registers inside the device. Likewise this input can be used in the ALU to modify any of the internal data files. The Q register is a separate 4-bit file intended primarily for multiplication and division routines but it can also be used as an accumulator or holding register for some applications. The ALU itself is a high speed arithmetic/logic operator capable of performing three binary arithmetic and five logic functions. The I3, I4, and I5 microinstruction inputs are used to select the ALU function. The definition of these inputs is shown in Figure 3. The octal code is also shown for reference. The normal technique for cascading ALU of several devices is in a look-ahead carry mode. Carry generate, GN, and carry propagate, PN, are outputs of the device for use with a carry- look-ahead-generator. A carry-out Cn + 4, is also generated and is available as an output for use as the carry flag in a status register. Both carry-in (Cn) and carry-out (Cn+4) are active HIGH. The ALU has three other status-oriented outputs. These are F 3, F=0, and overflow (OVR). The F3 output is the most significant (sign) bit of the ALU and can be used to determine positive or negative results without enabling the three-state data outputs. F3 is non-inverted with respect to the sign bit output Y3. The F = 0 output is used for zero detect. It is an open-collector output and can be wire OR’ed between microprocessor slices. F = 0 is HIGH when all F outputs are LOW. The overflow output (OVR) is used to flag arithmetic operations that exceed the available two’s complement number range. The overflow output (OVR) is HIGH when overflow exists. That is when Cn + 3 and Cn + 4 are not the same polarity. The ALU data output is routed to several destinations. It can be a data output of the device and it can also be stored in the RAM or the Q register. Eight possible combinations of ALU destination functions are available as defined by the I6, I7, and I8 microinstruction inputs. These combinations are shown in figure 4. The four-bit data output field (Y) features three-state outputs and can be directly bus organised. An output control (OEN) is used to enable the three-state outputs. When OEN is HIGH, the Y outputs are in the high impedance state. A two-input multiplexer is also used at the data output such that either the A-port of the RAM or the ALU outputs (F) are selected at the device Y outputs. This selection is controlled by the I6, I7, and I8 microinstruction inputs. As was discussed previously, the RAM inputs are driven from a three-input multiplexer. This allows the ALU outputs to be entered non-shifted, shifted up one position (x 2) or shifted down one position (÷ 2). The shifter has two ports; labeled RAM0 and RAM3. Both of these ports consist of a buffer-driver with a three-state output and an input to the multiplexer. Figure 2: ALU Source Operand Control Microcode ALU Source Operands R S Octal Code L L L 0 A C L L H 1 A B L H L 2 0 Q L H H 3 0 B H L L 4 0 A H L H 5 D A H H L 6 D Q H H H 7 D 0 I2 I1 I0 Microcode Octal Code ALU Function Symbol I5 I4 I3 L L L L H H H H L L H H L L H H L H L H L H L H 0 1 2 3 4 5 6 7 R plus S S minus R R minus S R OR S RN AND S R AND S R EX-OR S R EX-NOR S R + S S - R R - S R ∨ S RN ∧ S R ∧ S R ∇ S RN ∇ SN + = plus; - = minus; V = OR; Λ = AND; ∇ = EX-OR Figure 2: ALU Function Control |
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