Propeller Manual

3: Assembly Language Reference – RDBYTE
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 7..22
Concise Truth Table:
In Out
Destination Source Z C Effects Destination
1
Z
2
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 7 to 22 clock cycles to execute
depending on the relation between the cog’s hub access window and the instruction’s moment
of execution. See Hub on page 24 for more information.
Propeller Manual v1.1 · Page 335