Shift and rotate instructions

DSL (double shift left) is a multiply instruction, not a shift and rotate instruction.

Dauug|36’s shift instructions have unconventional terminology.

Arithmetic shift means multiplication or division by a power of two, rounding towards negative infinity in the case of division. In traditional forums, arithmetic shift means that a signed number is being shifted. But my position is that it’s moot whether the number is unsigned or unsigned. What is important is whether the intent is to perform arithmetic, and if this is the intent, the result must be tested for overrange irrespective of signedness.

Logical shift means a shift in the absence of any intent to multiply or divide. Logical shifts are not tested for overflow, because the bits involved do not represent quantities.

The architecture does not need or have an ROR instruction to rotate right, because ROL does that just fine.

ASL Arithmetic shift left

The 2-instruction PSL (prepare to shift left) and ASL (arithmetic shift left) sequence enables safe, range-checked power-of-two multiplication, despite its many signedness combinations and corner cases. ASL multiplies left operand a by a non-negative power of two, and writes the product’s 36 least significant bits to destination c. If the full result does not fit in c without loss of information, the T and R flags will be set. Otherwise, T is cleared and R does not change.

The PSL macro is used to convert the desired exponent of two into control word cw. This exponent is clamped to a maximum of 36, and then copied into all tribbles.

Note: PSL, like all macros, is not yet implemented. Construct cw another way.

ASR Arithmetic shift right

The 2-instruction PSR (prepare to shift right) and ASR (arithmetic shift right) sequence enables safe, range-checked division by powers of two, despite its many signedness combinations and corner cases. ASR divides left operand a by a non-negative power of two with rounding towards negative infinity, and writes the quotient’s 36 least significant bits (the 36 bits left of the radix mark, that is, the integer part of the quotient) to destination c. If the rounded quotient does not fit in c without loss of information, the T and R flags will be set, otherwise T is cleared and R does not change.

Note that because of the rounding towards negative infinity, ASR is not (for most purposes) a stand-alone means for dividing by powers of two numbers that are or may be negative.

The PSR macro is used to convert the desired exponent of two into control word cw. If the exponent is 36 or more, it is clamped to 36. If the exponent is zero, it is left as zero. Otherwise, the exponent is subtracted from 36 in order to represent it from the internal hardware perspective of a left rotation. After this clamping, leaving as zero, or subtracting, the exponent is copied to all tribbles.

Note: PSR, like all macros, is not yet implemented. Construct cw another way.

LSL Logical shift left

LSL (logical shift left) shifts the bits of register a left. Bits shifted out are discarded, and bits shifted in are zeros. The number of positions to shift may not be negative, and must be copied into every tribble of control word cw.

The PSL macro can convert any unsigned value into a suitable control word. N and Z are set as if the destination is a signed register. T and R are not changed by LSL or any preceding PSL.

Note: PSL, like all macros, is not yet implemented. Construct cw another way.

LSR Logical shift right

LSR (logical shift right) shifts the bits of register a right. Bits shifted out are discarded, and bits shifted in are zeros. The number of positions to shift may not be negative, is specified from the perspective of a left rotation, and must be copied into every tribble of control word cw.

The PSR macro can convert any unsigned value into a suitable control word. N and Z are set as if the destination is a signed register. T and R are not changed by LSR or any preceding PSR.

Note: PSR, like all macros, is not yet implemented. Construct cw another way.

ROL Rotate left

ROL (rotate left) rotates the bits of a. The number of positions to rotate must be specified from the perspective of a left rotation, may not be negative, and must be copied into every tribble of control word cw. The PRL and PRR macros offer a range-checked mechanism to set up the control word for left and right rotations. N and Z are set as if the destination is a signed register. ROL will not change T or R, although a preceding PRL and PRR may.

Unlike left and right shifts, left and right rotations are limited to 63 positions due to their repetition modulo 36. Evaluating a shift of more than 63 positions modulo 36 is computationally expensive and is not directly supported by Dauug|36. This is why PRL and PRR have to include a range check.

Again, there is no ROR instruction to rotate right. ROL is the instruction to rotate right or left. Remember also that the amount to rotate by is never negative.

Note: PRL and PRR, like all macros, is not yet implemented. Construct cw another way.