Useful macros for bit reversing and selecting in different ways. Only valid to be used from assembly language (not from C).
| Reverse Bits | Useful macros for bit reversing and selecting in different ways. |
| Macros | |
| cpctm_reverse_and_select_bits_of_A | Reorders the bits of A and mixes them letting the user select the new order for the bits by using a selection mask. |
| cpctm_reverse_bits_of_A | Macro: cpctm_reverse_mode_2_pixels_of_A |
| cpctm_reverse_mode_1_pixels_of_A | Reverses the order of pixel values contained in register A, assuming A is in screen pixel format, mode 1. |
| cpctm_reverse_mode_0_pixels_of_A | Reverses the order of pixel values contained in register A, assuming A is in screen pixel format, mode 0. |
Reorders the bits of A and mixes them letting the user select the new order for the bits by using a selection mask.
| TReg | An 8-bits register that will be used for intermediate calculations. This register may be one of these: B, C, D, E, H, L |
| SelectionMask | An 8-bits mask that will be used to select the bits to get from the reordered bits. It might be an 8-bit register or even (hl). |
| A | Byte to be reversed |
| TReg | Should have a copy of A (same exact value) |
| A | Resulting value with bits reversed and selected |
This macro reorders the bits in A and mixes them with the same bits in their original order by using a SelectionMask. The process is as follows:
1. Consider the 8 bits of A = TReg = [01234567] 2. Reorder the 8 bits of A, producing A2 = [32547610] 2. Reorder the bits of TReg, producing TReg2 = [76103254] 3. Combines both reorders into final result using a SelectionMask. Each 0 bit from the selection mask means “select bit from A2”, whereas each 1 bit means “select bit from TReg2”.
A2 = [ 32 54 76 10 ] TReg2 = [ 76 10 32 54 ] SelMask = [ 11 00 11 00 ] // 1 = TReg2-bits, 0 = A2-bits --------------------------- Result = [ 76 54 32 10 ]
Therefore, mask 0b11001100 produces the effect of reversing the bits of A completely. Other masks will produce different reorders of the bits in A, for different requirements or needs.
AF, TReg
16 bytes
Case | microSecs(us) | CPU Cycles ------------------------------------ Any | 16 | 64 ------------------------------------
Macro: cpctm_reverse_mode_2_pixels_of_A
Reverses the 8-bits of A, from [01234567] to [76543210]. This also reverses all pixels contained in A when A is in screen pixel format, mode 2.
| TReg | An 8-bits register that will be used for intermediate calculations. This register may be one of these: B, C, D, E, H, L |
| A | Byte to be reversed |
| TReg | Should have a copy of A (same exact value) |
| A | Resulting value with bits reversed |
This macro reverses the bits in A. If bits of A = [01234567], the final result after processing this macro will be A = [76543210]. Register TReg is used for intermediate calculations and its value is destroyed.
AF, TReg
16 bytes
Case | microSecs(us) | CPU Cycles ------------------------------------ Any | 16 | 64 ------------------------------------
Reverses the order of pixel values contained in register A, assuming A is in screen pixel format, mode 1.
| TReg | An 8-bits register that will be used for intermediate calculations. This register may be one of these: B, C, D, E, H, L |
| A | Byte with pixel values to be reversed |
| TReg | Should have a copy of A (same exact value) |
| A | Resulting byte with the 4 pixels values reversed in order |
This macro considers that A contains a byte that codifies 4 pixels in screen pixel format, mode 1. It modifies A to reverse the order of its 4 contained pixel values left-to-right (1234 -> 4321). With respect to the order of the 8-bits of A, the concrete operations performed is:
A = [012345678] == reverse-pixels ==> [10326587] = A2
You may want to check cpct_px2byteM1 to know how bits codify both pixels in one single byte for screen pixel format, mode 1.
TReg is an 8-bit register that will be used for intermediate calculations, destroying its original value (that should be same as A, at the start).
AF, TReg
16 bytes
Case | microSecs(us) | CPU Cycles ------------------------------------ Any | 16 | 64 ------------------------------------
Reverses the order of pixel values contained in register A, assuming A is in screen pixel format, mode 0.
| TReg | An 8-bits register that will be used for intermediate calculations. This register may be one of these: B, C, D, E, H, L |
| A | Byte with pixel values to be reversed |
| TReg | Should have a copy of A (same exact value) |
| A | Resulting byte with the 2 pixels values reversed in order |
This macro considers that A contains a byte that codifies 2 pixels in screen pixel format, mode 0. It modifies A to reverse the order of its 2 contained pixel values left-to-right (12 -> 21). With respect to the order of the 8-bits of A, the concrete operation performed is:
A = [012345678] == reverse-pixels ==> [10325476] = A2
You may want to check cpct_px2byteM0 to know how bits codify both pixels in one single byte for screen pixel format, mode 0.
TReg is an 8-bit register that will be used for intermediate calculations, destroying its original value (that should be same as A, at the start).
AF, TReg
7 bytes
Case | microSecs(us) | CPU Cycles ------------------------------------ Any | 7 | 28 ------------------------------------