Summary
Highlights
A recap of MIPS ISA was provided, covering ALU, logical, and arithmetic instructions with three register operands or immediate (constant) values. Data transfer instructions (load/store) for memory access, control instructions for branching (BEQ, BNE, J), and support for various data operand sizes (byte, half-word, word) and unsigned numbers were reviewed. The three instruction formats (R, I, J) and the MIPS reference card for characteristics were also mentioned.
The process of translating a C if-else statement into MIPS assembly was detailed. The primary decision involves using BRANCH instructions (BEQ or BNE). If the condition is equality, a BNE (Branch Not Equal) is used to jump to the 'else' block, executing the 'if' block sequentially. An unconditional JUMP instruction is then used to skip the 'else' block and continue program flow. Labels are used to identify instruction sequences, which are translated into memory addresses during compilation.
The implementation of a while loop in MIPS assembly was explained, focusing on array (vector) access. Calculating memory addresses for array elements involves multiplying the index by 4 (bytes per word), which is efficiently done using SL (Shift Left Logical) instruction rather than MUL. The loop condition is checked using a comparison, followed by a BNE to exit the loop or an ADDI instruction to increment the loop variable. An unconditional JUMP returns to the beginning of the loop.
The video discusses how MIPS handles comparisons for greater than/less than conditions, which are more complex than equality in hardware. MIPS uses 'Set Less Than' (SLT) instructions, which set a register to 1 if the condition is true and 0 if false. This result can then be used with a conditional branch. The crucial difference between signed (SLT) and unsigned (SLTU) comparisons was highlighted, emphasizing that the interpretation of bits significantly alters comparison outcomes, thus requiring appropriate instruction choice.
The convention for handling procedure (function) calls in MIPS was outlined. Parameters are passed using reserved registers ($a0-$a3). The JAL (Jump And Link) instruction is used to call a procedure, saving the return address in the $ra register. Inside the procedure, 'caller-saved' registers ($t0-$t9) can be modified freely, while 'callee-saved' registers ($s0-$s7) must be saved onto the stack before use and restored before returning. Results are returned in $v0 and $v1 registers. The stack pointer ($sp) is adjusted for saving and restoring registers. Finally, JR $ra is used to return to the caller.
Five MIPS addressing modes were summarized: immediate addressing (constant value embedded in instruction), register addressing (operand in a register), base addressing (register content + offset for memory access, common for load/store), PC-relative addressing (PC + offset for conditional branches), and pseudodirect addressing (26-bit address from instruction combined with upper bits of PC for unconditional jumps).
The importance of synchronization for atomic operations, such as swapping two register values in a multi-process environment, was discussed. MIPS provides special instructions, Load Linked (LL) and Store Conditional (SC), to ensure atomicity. LL loads a value and monitors the memory location for changes. SC attempts to store a value; it succeeds only if the monitored location hasn't changed since the LL. If it fails, the operation must be retried. This mechanism guarantees that read-modify-write operations are atomic.