GIFtags

The basic unit of GIF transfer data is the GIF packet. Each packet is split into one or more primitives. Every primitive must be preceded by a GIFtag.

GIFtag Format

• 0-14 NLOOP - Data per register to transfer
• 15 EOP - End of packet
• 16-45 Unused
• 46 Enable PRIM field
• 47-57 Data to be sent to GS PRIM register if GIFtag.46 == 1
• 58-59 Data format
  • 0=PACKED
  • 1=REGLIST
  • 2=IMAGE
  • 3=IMAGE
• 60-63 NREGS - Number of registers
  • 0=16 registers
• 64-127 Register field, 4 bits each

When NLOOP == 0, all fields are ignored except EOP and no further processing is done. When GIFtag.46 == 0, an idle cycle is inserted before processing begins. Registers are handled in little-endian order; i.e. bits 64-67 are processed first, then 68-71, and so on.