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\section{Control Flow Instructions}
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Control flow instruction load immediate values or register values into the \pc and/or load the value of the \pc into a general-purpose register. The \eco supports unconditional {\it jumps}, conditional {\it branches}, indirect jumps, subroutine calls, subroutine returns, and indirect subroutine calls out of the box. More complex control flow schemes can be implemented by combining these instructions.
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A control transfer is \definition{conditional} if it only occurs on a certain condition that is computed from general-purpose registers. A control transfer is \definition{unconditional} if it always occurs.
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A control transfer is \definition{direct} if the target address is supplied as an immediate value. It is \definition{indirect} if the target address is supplied as a register value.
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A control transfer is \definition{absolute} if the value of the \pc is overwritten with a totally new value. It is \definition{relative} if the value of the \pc is modified by adding or subtracting an offset.
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Both relative control transfers and instructions that read the current \pc value operate on the value of the \pc {\it after} increasing it by 4 during instruction fetching.
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\newcommand{\branchdesc}[4]{
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\subsection{#1}
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The #1 instruction performs a conditional direct jump to a relative immediate sign-extended 16-bit offset counted as words. The condition is evaluated by comparing two 32-bit register operands and is asserted if the first operand is #3 the second operand.
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\brformat{#2}
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\begin{effectize}
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\effect if $R_x #4 R_y$ then $PC \leftarrow PC + 4 * signext_{32}(offset)$
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\end{effectize}
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}
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\branchdesc{BEQ}{100000}{equal to}{=}
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\branchdesc{BNE}{100001}{not equal to}{\neq}
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\branchdesc{BLE}{100010}{less or equal to (by signed comparison)}{\leq_{signed}}
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\branchdesc{BLEU}{100011}{less or equal to (by unsigned comparison)}{\leq_{unsigned}}
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\branchdesc{BLT}{100100}{less than (by signed comparison)}{<_{signed}}
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\branchdesc{BLTU}{100101}{less than (by unsigned comparison)}{<_{unsigned}}
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\branchdesc{BGE}{100110}{greater or equal to (by signed comparison)}{\geq_{signed}}
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\branchdesc{BGEU}{100111}{greater or equal to (by unsigned comparison)}{\geq_{unsigned}}
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\branchdesc{BGT}{101000}{greater than (by signed comparison)}{>_{signed}}
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\branchdesc{BGTU}{101001}{greater than (by unsigned comparison)}{>_{unsigned}}
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\subsection{J}
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The J instruction performs an unconditional direct jump to a relative immediate sign-extended 26-bit offset counted as words.
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\jformat{101010}
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\begin{effectize}
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\effect $PC \leftarrow PC + 4 * signext_{32}(offset)$
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\end{effectize}
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\subsection{JR}
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The JR instruction performs an unconditional indirect jump to an absolute offset stored in a general-purpose register. It can be used for simple indirect jumps as well as to return from a subroutine.
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\jrformat{101011}
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\begin{effectize}
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\effect $PC \leftarrow R_{dest}$
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\end{effectize}
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\subsection{JAL}
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The JAL instruction stores the current \pc value in register \#31, then performs an unconditional direct jump to a relative immediate sign-extended 26-bit offset counted as words. It is primarily used for subroutine calls.
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\jformat{101100}
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\begin{effectize}
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\effect $R_{31} \leftarrow PC$
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\effect $PC \leftarrow PC + 4 * signext_{32}(offset)$
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\end{effectize}
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\subsection{JALR}
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The JALR instruction remembers the current \pc value, then performs an unconditional indirect jump to an absolute offset stored in a general-purpose register. The previous PC value is then stored in register \#31. It is primarily used for indirect subroutine calls, such as virtual method invocations in object-oriented programming.
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\jrformat{101101}
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\begin{effectize}
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\effect $returnAddress \leftarrow PC$
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\effect $PC \leftarrow R_{dest}$
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\effect $R_{31} \leftarrow returnAddress$
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\end{effectize}
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