RDBYTE
Instruction: Read byte of main memory.
RDBYTE Value, < # > Address
Result: Zero-extended byte is stored in Value.
- Value (d-field) is the register to store the zero-extended byte value into.
- Address (s-field) is a register or a 9-bit literal whose value is the main memory address to read from.
Opcode Table:
–INSTR– ZCRI –CON– –DEST– –SRC– | Z Result | C Result | Result | Clocks |
000000 001i 1111 ddddddddd sssssssss | Result = 0 | — | Written | 8..23 |
Concise Truth Table:
In | Out | ||||||
Destination | Source | Z | C | Effects | Destination1 | Z2 | C |
| $----_----; - | $----_----; - | - | - | wz wc | 31:8 = 0, 7:0 = byte value | 0 | 0 |
1 Destination Out is the zero-extended byte value read from main memory and is always generated since including an NR effect would turn RDBYTE into a WRBYTE instruction.
2 The Z flag is cleared (0) unless Destination Out equals 0.
Explanation
RDBYTE syncs to the Hub, reads the byte of main memory at Address, zero-extends it, and stores it into the Value register.
If the WZ effect is specified, the Z flag will be set (1) if the value read from main memory is zero. The NR effect can not be used with RDBYTE as that would change it to a WRBYTE instruction.
RDBYTE is a hub instruction. Hub instructions require 8 to 23 clock cycles to execute depending on the relation between the cog's hub access window and the instruction's moment of execution. See Hub for more information.
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